Light source and display system incorporating same

ABSTRACT

Light sources are disclosed. A disclosed light source includes an optically reflective cavity that includes an input port for receiving light and an output port for transmitting light, a lamp that is disposed at the input port, and an optical stack that is disposed at the output port. The optical stack includes a forward scattering optical diffuser that is disposed at the output port and has an optical haze that is not less than about 20%, and an optical film that is disposed on the optical diffuser. The optical film enhance total internal reflection at the interface between the optical film and the optical diffuser. The optical film has an index of refraction that is not greater than about 1.3 and an optical haze that is not greater than about 5%. The optical stack also includes a reflective polarizer layer that is disposed on the optical film. Substantial portions of each two neighboring major surfaces in the optical stack are in physical contact with each other.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/502,060, filed on Apr. 13, 2012, which is a national stage filingunder 35 U.S.C. 371 of PCT/US2010/053719, filed on Oct. 22, 2010, whichclaims priority to U.S. Provisional Application No. 61/254,672, filed onOct. 24, 2009, the disclosure of which is incorporated by reference inits/their entirety herein.

RELATED APPLICATIONS

This application is related to the following U.S. patent applicationswhich are incorporated herein in their entireties by reference: “OpticalFilm” filed on Apr. 15, 2009 and having Ser. No. 61/169,466; “OpticalConstruction and Display System Incorporating Same” filed on Apr. 15,2009 and having Ser. No. 61/169,521; “Retroreflecting OpticalConstruction” filed on Apr. 15, 2009 and having Ser. No. 61/169,532;“Optical Film for Preventing Optical Coupling” filed on Apr. 15, 2009and having Ser. No. 61/169,549; “Backlight and Display SystemIncorporating Same” filed on Apr. 15, 2009 and having Ser. No.61/169,555; “Process and Apparatus for Coating with Reduced Defects”filed on Apr. 15, 2009 and having Ser. No. 61/169,427; “Process andApparatus for A Nanovoided Article” filed on Apr. 15, 2009 and havingSer. No. 61/169,429; and “Optical Construction and Method of Making theSame” filed on Oct. 22, 2009 and having Ser. No. 61/254,243.

This application is further related to the following U.S. patentapplications, filed on even date herewith and which are incorporatedherein in their entireties by reference: “Gradient Low Index Article andMethod” having Ser. No. 61/254,673; “Process for Gradient NanovoidedArticle” having Ser. No. 61/254,674; “Immersed Reflective Polarizer withHigh Off-Axis Reflectivity” having Ser. No. 61/254,691; “ImmersedReflective Polarizer With Angular Confinement in Selected Planes ofIncidence” having Ser. No. 61/254,692; and “Voided Diffuser” having Ser.No. 61/254,676.

FIELD OF THE INVENTION

This invention generally relates to light sources that include a hollowoptically reflective cavity and an optical film that exhibits somelow-refractive index-like properties. The invention also relates toillumination devices, backlights and display systems that incorporatesuch light sources.

BACKGROUND

Backlights are used as extended area illumination sources in displayssuch as liquid crystal displays (LCDs). Backlights typically incorporatea light source that includes one or more lamps, a lightguide forproducing an extended area light source by extending light from thelamps over the output surface of the backlight, and one or more lightmanagement layers such as prismatic light redirecting layers, brightnessenhancement layers, reflective polarizer layers, diffuser layers, mirrorlayers and retarder layers. Lightguides are typically solid and includemeans for extracting light from the lightguide.

SUMMARY OF THE INVENTION

Generally, the present inventions relates to light sources. In oneembodiment, a light source includes a reflective cavity that includes aninput port for receiving light and an output port for transmittinglight. The light source also includes a lamp that is disposed at theinput port. The light source also includes an optical stack that isdisposed at the output port and includes a substantially forwardscattering optical diffuser that is disposed at the output port and hasan optical haze that is not less than about 20%, an optical film that isdisposed on the optical diffuser for enhancing total internal reflectionat the interface between the optical film and the optical diffuser. Theoptical film has an index of refraction that is not greater than about1.3 and an optical haze that is not greater than about 5%. The opticalstack also includes a reflective polarizer layer that is disposed on theoptical film. Substantial portions of each two neighboring majorsurfaces in the optical stack are in physical contact with each other.In some cases, the ratio of the maximum lateral dimension of theoptically reflective cavity to the maximum thickness of the opticallyreflective cavity is not less than about 20, or not less than about 40,or not less than about 60. In some cases, the lamp includes an LED. Insome cases, the cavity includes input ports on opposite sides of thecavity. In some cases, the output port of the cavity is located on a topside of the cavity. In some cases, the optical diffuser has a transportratio that is not less than about 0.2, or not less than about 0.3, ornot less than about 0.4, or not less than about 0.5. In some cases, theoptical diffuser is a semi-specular partial reflector. In some cases,the optical haze of the optical diffuser is not less than about 30%, ornot less than about 40%. In some cases, the optical diffuser includes asurface diffuser, or a volume diffuser, or a combination of a volumediffuser and a surface diffuser. In some cases, the effective index ofrefraction of the optical film is not greater than about 1.25, or notgreater than about 1.2, or not greater than about 1.15, or not greaterthan about 1.1. In some cases, the optical haze of the optical film isnot greater than about 4%, or not greater than about 3%, or not greaterthan about 2%. In some cases, the optical film includes a plurality ofinterconnected voids, and in some cases, the optical film also includesparticles that can, for example, be or include fumed silica. In somecases, the optical film is laminated to the optical diffuser via anoptical adhesive layer. In some cases, the optical film is coated on thereflective polarizer layer. In some cases, the optical stack includes anoptical adhesive layer that is disposed on the reflective polarizerlayer. In some cases, the reflective polarizer layer includes amultilayer optical film wherein at least some of the layers arebirefringent. In some cases, the reflective polarizer layer includes awire grid reflective polarizer, or a reflective fiber polarizer, or acholesteric reflective polarizer, or a diffusely reflective polarizingfilm (DRPF). In some cases, the optically reflective cavity includes oneor more specularly reflective side reflectors for at least partiallycollimating light that is emitted by the lamps. In some cases, thecavity includes a specularly reflective back reflector that faces theoutput port. In some cases, at least 50%, or at least 70%, or at least90%, of each two neighboring major surfaces in the optical stack are inphysical contact with each other. In some cases, the optical film isdisposed between the reflective polarizer layer and the opticaldiffuser. In some cases, the light source is included in a backlight ina display system.

In another embodiment, a light source includes a reflective cavity thatincludes an input port for receiving light and an output port fortransmitting light, a lamp disposed at the input port, and an opticalstack that is disposed at the output port and includes an optical filmthat is disposed at the output port and has an optical haze that is notless than about 30%, and a reflective polarizer layer that is disposedon the optical film, where substantial portions of each two neighboringmajor surfaces in the optical stack are in physical contact with eachother. In some cases, the ratio of the maximum lateral dimension of thecavity to the maximum thickness of the cavity is not less than about 20,or not less than about 40, or not less than about 60. In some cases, thelamp includes an LED. In some cases, the cavity includes input portslocated on opposite sides of the cavity. In some cases, the output portof the cavity is located on the top side of the cavity. In some cases,the optical film has a transport ratio that is not less than about 0.2,or not less than about 0.3, or not less than about 0.4, or not less thanabout 0.5. In some cases, the optical haze of the optical film is notless than about 40%, or not less than about 50%. In some cases, theoptical film includes a binder, a plurality of interconnected voids, anda plurality of particles, where the particles can include fumed silica.In some cases, the optical film is laminated to the reflective polarizerlayer via an optical adhesive layer. In some cases, the optical film isdirectly coated on the reflective polarizer layer. In some cases, theoptical stack includes an optically adhesive layer that is disposed onthe reflective polarizer layer. In some cases, the reflective polarizerlayer includes a multilayer optical film, or a wire grid reflectivepolarizer, or reflective fiber polarizer, or a cholesteric reflectivepolarizer, or a diffusely reflective polarizing film (DRPF). In somecases, the cavity comprises a specularly reflective side reflector atleast partially collimating light that is emitted by the lamp. In somecases, the cavity includes a specularly reflective back reflector thatfaces the output port. In some cases, at least 50%, or at least 70%, orat least 90%, of each two neighboring major surfaces in the opticalstack are in physical contact with each other.

In another embodiment, a light source includes an optically reflectivecavity that includes an input port for receiving light and an outputport for transmitting light, a lamp that is disposed at the input ports,and an optical stack that is disposed at the output port and includes anoptical diffuser that is disposed at the output port and has an opticalhaze that is not less than about 20% and an optical film that isdisposed on the optical diffuser for enhancing total internal reflectionat the interface between the optical film and the optical diffuser. Theoptical film has an index of refraction that is not greater than about1.3 and an optical haze that is not greater than about 5%. The opticalstack also includes a partially reflective partially transmissive layerthat is disposed on the optical film. Substantial portions of each twoneighboring major surfaces in the optical stack are in physical contactwith each other.

In another embodiment, a light source includes an optically reflectivehollow cavity that includes an input port for receiving light, a firstoutput port for transmitting light, a second output port fortransmitting light, and a lamp that is disposed at the input ports, afirst optical stack that is disposed at the first output port, and adifferent second optical stack that is disposed at the second outputport. At least one of the optical stacks includes an optical film thathas an optical haze that is not less than about 30%, and a reflectivepolarizer layer that is disposed on the optical film, where substantialportions of each two neighboring major surfaces in the optical stack arein physical contact with each other. In some cases, a display systemincludes a first liquid crystal panel that is disposed on the firstoptical stack, and a second liquid crystal panel that is disposed on thesecond optical stack.

BRIEF DESCRIPTION OF DRAWINGS

The invention may be more completely understood and appreciated inconsideration of the following detailed description of variousembodiments of the invention in connection with the accompanyingdrawings, in which:

FIG. 1 is a schematic side-view of a display system;

FIG. 2 is a schematic illustration of forward and backward scattering;

FIG. 3 is a schematic side-view of another display system;

FIG. 4 is a schematic side-view of an optical stack;

FIG. 5 is a schematic side-view of a display system;

FIG. 6 is a schematic side-view of a light source;

FIG. 7 is a schematic side-view of an optical stack;

FIG. 8 is a schematic side-view of another display system;

FIG. 9 is a grayscale conoscopic image of the measured luminance of adisplay system as a function of viewing angle;

FIG. 10 is a schematic side-view of an optical stack;

FIG. 11 is a grayscale conoscopic image of the measured luminance ofanother display system as a function of viewing angle;

FIG. 12 is a schematic side-view of a display system;

FIG. 13 is a schematic side-view of another display system;

FIG. 14 is a schematic side-view of another display system;

FIG. 15 is a schematic side-view of an optical stack;

FIG. 16 is a schematic side-view of another optical stack;

FIG. 17 is a schematic side-view of a display system;

FIG. 18 is a schematic side-view of another display system;

FIG. 19 is a schematic side-view of an optical construction;

FIG. 20 is a schematic side-view of another optical construction; and

FIG. 21 is a schematic side-view of an optical stack.

In the specification, a same reference numeral used in multiple figuresrefers to the same or similar elements having the same or similarproperties and functionalities.

DETAILED DESCRIPTION

This invention generally relates to light sources that include a hollowreflective cavity and an optical film that has a low effective index ofrefraction or exhibits some low-refractive index-like properties. Insome cases, the disclosed light sources are extended light sources andcan advantageously be incorporated into displays, such as liquid crystaldisplays (LCDs), to provide extended illumination to an image formingpanel. Displays incorporating the disclosed light sources can havereduced thickness and weight. The disclosed light sources can utilizefewer lamps and provide uniform illumination over an extend area byefficient light mixing.

The disclosed light sources include an optical film that, in some cases,has a low optical haze and a low effective index of refraction, such asan optical haze of less than about 5% and an effective index ofrefraction that is less than about 1.3. In some cases, the optical filmhas a high optical haze and/or high diffuse optical reflectance whileexhibiting some low-refractive-index-like optical properties, such as,for example, the ability to support total internal reflection or enhanceinternal reflection.

The optical films disclosed herein include a plurality of voids, such asa plurality of interconnected voids or a network of voids, dispersed ina binder. The voids in the plurality of interconnected voids areconnected to one another via hollow tunnels or hollow tunnel-likepassages. The voids are not necessarily free of all matter and/orparticulates. For example, in some cases, a void may include one or moresmall fiber- or string-like objects that include, for example, a binderand/or nano-particles. Some disclosed optical films include multiplepluralities of interconnected voids or multiple networks of voids wherethe voids in each plurality or network are interconnected. In somecases, in addition to multiple pluralities of interconnected voids, thedisclosed optical films include a plurality of closed or unconnectedvoids meaning that the voids are not connected to other voids viatunnels.

Some disclosed optical films support total internal reflection (TIR) orenhanced internal reflection (EIR) by virtue of including a plurality ofvoids. When light that travels in an optically clear non-porous mediumis incident on a stratum possessing high porosity, the reflectivity ofthe incident light is much higher at oblique angles than at normalincidence. In the case of no or low haze voided films, the reflectivityat oblique angles greater than the critical angle is close to about100%. In such cases, the incident light undergoes total internalreflection (TIR). In the case of high haze voided films, the obliqueangle reflectivity can be close to 100% over a similar range of incidentangles even though the light may not undergo TIR. This enhancedreflectivity for high haze films is similar to TIR and is designated asEnhanced Internal Reflectivity (EIR). As used herein, by a porous orvoided optical film enhancing internal reflection (EIR), it is meantthat the reflectance at the boundary of the voided and non-voided strataof the film or film laminate is greater with the voids than without thevoids.

The voids in the disclosed optical films have an index of refractionn_(v) and a permittivity ε_(v), where n_(v) ²=ε_(v), and the binder hasan index of refraction n_(b) and a permittivity ε_(b), where n_(b)²=ε_(b). In general, the interaction of an optical film with light, suchas light that is incident on, or propagates in, the optical film,depends on a number of film characteristics such as, for example, thefilm thickness, the binder index, the void or pore index, the pore shapeand size, the spatial distribution of the pores, and the wavelength oflight. In some cases, light that is incident on or propagates within theoptical film, “sees” or “experiences” an effective permittivity ε_(eff)and an effective index n_(eff), where n_(eff) can be expressed in termsof the void index n_(v), the binder index n_(b), and the film porosityor void volume fraction “f”. In such cases, the optical film issufficiently thick and the voids are sufficiently small so that lightcannot resolve the shape and features of a single or isolated void. Insuch cases, the size of at least a majority of the voids, such as atleast 60% or 70% or 80% or 90% of the voids, is not greater than aboutλ/5, or not greater than about λ/6, or not greater than about λ/8, ornot greater than about λ/10, or not greater than about λ/20, where λ isthe wavelength of light.

In some cases, light that is incident on a disclosed optical film is avisible light meaning that the wavelength of the light is in the visiblerange of the electromagnetic spectrum. In such cases, the visible lighthas a wavelength that is in a range from about 380 nm to about 750 nm,or from about 400 nm to about 700 nm, or from about 420 nm to about 680nm. In such cases, the optical film can reasonably be assigned aneffective index of refraction if the size of at least a majority of thevoids, such as at least 60% or 70% or 80% or 90% of the voids, is notgreater than about 70 nm, or not greater than about 60 nm, or notgreater than about 50 nm, or not greater than about 40 nm, or notgreater than about 30 nm, or not greater than about 20 nm, or notgreater than about 10 nm.

In some cases, the disclosed optical films are sufficiently thick sothat the optical film can reasonably have an effective index that can beexpressed in terms of the indices of refraction of the voids and thebinder, and the void or pore volume fraction or porosity. In such cases,the thickness of the optical film is not less than about 100 nm, or notless than about 200 nm, or not less than about 500 nm, or not less thanabout 700 nm, or not less than about 1000 nm.

When the voids in a disclosed optical film are sufficiently small andthe optical film is sufficiently thick, the optical film has aneffective permittivity ε_(eff) that can be expressed as:ε_(eff) =fε _(v)+(1−f)ε_(b)  (1)

In such cases, the effective index n_(eff) of the optical film can beexpressed as:n _(eff) ² =fn _(v) ²+(1−f)n _(b) ²  (2)

In some cases, such as when the difference between the indices ofrefraction of the pores and the binder is sufficiently small, theeffective index of the optical film can be approximated by the followingexpression:n _(eff) =fn _(v)+(1−f)n _(b)  (3)

In such cases, the effective index of the optical film is the volumeweighted average of the indices of refraction of the voids and thebinder. For example, an optical film that has a void volume fraction ofabout 50% and a binder that has an index of refraction of about 1.5, hasan effective index of about 1.25.

FIG. 1 is a schematic side-view of a display system 1200 that includes aliquid crystal panel 1280 disposed on an extended light source 100.Light source 100 includes an optical stack 1290 that is disposed on andreceives light from an optically reflective cavity 1215.

Optically reflective cavity 1215 includes at least one specularlyreflective reflector, an input port for receiving light from a lamp, anoutput port for transmitting light, and means for improving collimationof light emitted by the lamp, where the improved collimation is in thexz-plane or along a lateral direction, such as along the length and/orwidth direction, of the optically reflective cavity. In particular,optically reflective cavity 1215 includes specularly reflective sidereflectors 1210A and 1210B and an input port 1204A on one (right) sideof the optical cavity, specularly reflective side reflectors 1210C and1210D and an input port 1204B on the opposite (left) side of the opticalcavity, lamps 1201 at input port 1204A, and lamps 1202 at input port1204B. Light that is emitted by lamps 1201 is collimated, or partiallycollimated, by specular side-reflectors 1210A and 1210B generally alongthe length (x−) direction of the optically reflective cavity. Similarly,light that is emitted by lamps 1202 is collimated, or partiallycollimated, by specular side-reflectors 1210C and 1210D generally alongthe length (x−) direction of the optically reflective cavity. Opticallyreflective cavity 1215 also includes an output port 1204C fortransmitting light that is emitted by the lamps, and a specularly backor bottom reflector 1212 on the back or bottom side of the cavity facingoutput port 1204C.

Optical stack 1290 includes a substantially forward scattering opticaldiffuser 1220 disposed at output port 1204C, a first optical adhesivelayer 1230 disposed on the optical diffuser, an optical film 1240disposed on the first optical adhesive layer, a reflective polarizerlayer 1250 disposed on the optical film, a second optical adhesive layer1235 disposed on the reflective polarizer layer, and a substrate 1260disposed on the second optical adhesive layer. Optical film 1240 isdisposed between reflective polarizer layer 1250 and the substantiallyforward scattering optical diffuser layer 1220.

Optical film 1240 has a sufficiently low refractive index and lowoptical haze and is sufficiently thick so as to promote or enhance totalinternal reflection at interface 1242 between optical film 1240 andfirst optical adhesive layer 1230. The index of refraction of theoptical film is not greater than about 1.3, or not greater than about1.25, or not greater than about 1.2, or not greater than about 1.15, ornot greater than about 1.1, or not greater than about 1.05. Thethickness of the optical film is not less than about 0.7 microns, or notless than about 0.8 microns, or not less than about 0.9 microns, or notless than about 1 micron, or not less than about 1.1 microns, or notless than about 1.2 microns, or not less than about 1.3 microns, or notless than about 1.4 microns, or not less than about 1.5 microns, or notless than about 1.7 microns, or not less than about 2 microns. Theoptical haze of the optical film is not greater than about 5%, or notgreater than about 4%, or not greater than about 3%, or not greater thanabout 2%, or not greater than about 1%, or not greater than about 0.5%.

For light normally incident on optical film 1240, optical haze, as usedherein, is defined as the ratio of the transmitted light that deviatesfrom the normal (y−) direction by more than 4 degrees to the totaltransmitted light. Haze values disclosed herein were measured using aHaze-Gard Plus haze meter (BYK-Gardiner, Silver Springs, Md.) accordingto the procedure described in ASTM D1003.

Optical film 1240 has a high optical clarity. For light normallyincident on optical film 120, optical clarity, as used herein, refers tothe ratio (T₂−T₁)/(T₂+T₁), where T₁ is the transmitted light thatdeviates from the normal direction between 1.6 and 2 degrees, and T₂ isthe transmitted light that lies between zero and 0.7 degrees from thenormal direction. Clarity values disclosed herein were measured using aHaze-Gard Plus haze meter from BYK-Gardiner. In the cases where opticalfilm 1240 has a high optical clarity, the clarity is not less than about50%, or not less than about 60%, or not less than about 70%, or not lessthan about 80%, or not less than about 90%, or not less than about 95%.

Optical film 1240 includes a plurality of voids, such as interconnectedvoids, dispersed in a binder. The binder can be or include any materialthat may be desirable in an application. For example, the binder can bea UV curable material that forms a polymer, such as a cross-linkedpolymer. In general, the binder can be any polymerizable material, suchas a polymerizable material that is radiation-curable.

In some cases, optical film 1240 also includes a plurality of particlesdispersed in the binder and/or the optical film. The particles can beany type particles that may be desirable in an application. For example,the particles in optical film 1240 can be organic or inorganicparticles. For example, the particles can be silica, zirconium oxide oralumina particles. The particles in optical film 1240 can have any shapethat may be desirable or available in an application. For example, theparticles can have a regular or irregular shape. For example, theparticles can be approximately spherical. As another example, theparticles can be elongated. In such cases, optical film 1240 includes aplurality of elongated particles. In some cases, the elongated particleshave an average aspect ratio that is not less than about 1.5, or notless than about 2, or not less than about 2.5, or not less than about 3,or not less than about 3.5, or not less than about 4, or not less thanabout 4.5, or not less than about 5. In some cases, the particles inoptical film 1240 can be in the form or shape of a string-of-pearls(such as Snowtex-PS particles available from Nissan Chemical, Houston,Tex.) or aggregated chains of spherical or amorphous particles, such asfumed silica.

The particles in optical film 1240 may or may not be functionalized. Insome cases, the particles are not functionalized. In some cases, theparticles are functionalized so that they can be dispersed in a desiredsolvent or binder with no, or very little, clumping. In some cases, theparticles can be further functionalized to chemically bond to the hostbinder. For example, the particles can be surface modified and havereactive functionalities or groups to chemically bond to the binder. Insome cases, some of the particles in optical film 1240 have reactivegroups and others do not have reactive groups. For example in somecases, about 10% of the particles have reactive groups and about 90% ofthe particles do not have reactive groups, or about 15% of the particleshave reactive groups and about 85% of the particles do not have reactivegroups, or about 20% of the particles have reactive groups and about 80%of the particles do not have reactive groups, or about 25% of theparticles have reactive groups and about 75% of the particles do nothave reactive groups, or about 30% of the particles have reactive groupsand about 60% of the particles do not have reactive groups, or about 35%of the particles have reactive groups and about 65% of the particles donot have reactive groups, or about 40% of the particles have reactivegroups and about 60% of the particles do not have reactive groups, orabout 45% of the particles have reactive groups and about 55% of theparticles do not have reactive groups, or about 50% of the particleshave reactive groups and about 50% of the particles do not have reactivegroups. In some cases, some of the particles can be functionalized withboth reactive and non-reactive groups. For example, in some cases, asubstantial fraction of the particles, such as at least about 30%, or atleast about 40%, or at least about 50%, or at least about 60%, or atleast about 70%, or at least about 80%, of the particles can befunctionalized with both reactive and non-reactive groups.

In some cases, the particles can have an average diameter of greaterthan about 0.5 microns, or greater than about 1 micron, or greater thanabout 1.5 microns, or greater than about 2 microns. In some cases, theparticles can have an average diameter that is less than about 1 micron,or less than about 0.7 microns, or less than about 0.5 microns, or lessthan about 0.3 microns, or less than about 0.2 microns, or less thanabout 0.1 microns, or less than about 0.07 microns, or less than about0.05 microns. In some cases, the optical film can have a first pluralityof larger particles that have an average diameter that is not less thanabout 1 micron and a second plurality of smaller particles that have anaverage diameter that is not greater than about 0.5 microns. In suchcases, the particle size distribution can have a first peak located atless than about 0.5 microns and a second peak located at greater thanabout 1 micron.

Optical film 1240 can be any optical film that includes a plurality ofvoids. For example, optical film 1240 can be an optical film describedin U.S. Patent Application Ser. No. 61/169,466 titled “Optical Film”,filed on Apr. 15, 2009, and U.S. Patent Application Ser. No. 61/169,521“Optical Construction and Display System Incorporating Same” filed onApr. 15, 2009. As another example, optical film 1240 can be an opticalfilm described in U.S. Patent Application Ser. No. 61/254,676 titled“Voided Diffuser”, and U.S. Patent Application Ser. No. 61/254,243“Optical Construction and Method of Making the Same”, the disclosures ofwhich are incorporated herein in their entireties by reference.

Optical diffuser 1220 is a substantially forward scattering diffusermeaning that a substantial portion, such as at least about 20%, or atleast about 30%, or at least about 40%, or at least about 50%, or atleast about 60%, or at least about 70%, or at least about 80%, of lightincident on the optical diffuser is scattered in a forward direction asforward scattered reflected and/or transmitted lights.

FIG. 2 is a schematic illustration of a light ray 205 that is incidentat an incident point 225 on an interface 210 between a first medium 215and a different second medium 220. Incident point 225 defines a normalplane 230 that is normal to interface 210 at incident point 225. Plane230 divides the space into a forward section 235 and a backward section240. Portions of incident light 205 that are scattered in a forwarddirection lie and propagate in forward section 235 and portions ofincident light 205 that are scattered in backward direction lie andpropagate in backward section 240. For example, light ray 205 isscattered at interface 210 resulting in a forward scattered transmittedlight 245 having a flux F1, a forward scattered reflected light 250having a flux F2, a backward scattered transmitted light 260 having aflux B1, and a backward scattered reflected light 255 having a flux B2.The total light scattered in the forward direction has a flux F=F1+F2and the total light scattered in the backward direction has a fluxB=B1+B2. The degree of forward scattering of incident light ray 205 byinterface 210 can be characterized by a “transport ratio” TR, definedas:TR=(F−B)/(F+B)  (4)where TR can, in general, have a value in a range from zero to one. Forexample, in the case of a specular reflector, F1, B1 and B2 are zeroresulting in a transport ratio of one. As another example, in the caseof a Lambertian reflector, F1 and B1 are zero, and F2=B2 resulting in atransport ratio of zero.

Referring back to FIG. 1, in some cases, such as when optical diffuserlayer 1220 is a substantially forward scattering optical diffuser, theoptical diffuser layer has a transport ratio that is not less than about0.2, or not less than about 0.3, or not less than about 0.4, or not lessthan about 0.5, or not less than about 0.6, or not less than about 0.8.

Optical diffuser layer 1220 transmits a portion of an incident light andreflects another portion of the incident light. In some cases, theoptical reflectance of optical diffuser layer 1220 is at least 40%, orat least 50%, or at least 60%, or at least 70%, or at least 80%, or atleast 90%. In some cases, the optical transmittance of optical diffuserlayer 1220 is not greater than about 30%, or not greater than about 25%,or not greater than about 20%, or not greater than about 15%, or notgreater than about 10%. In some cases, such as when light source 100provides uniform illumination, the optical haze of substantially forwardscattering optical diffuser layer 1220 is not less than about 20%, ornot less than about 30%, or not less than about 40%, or not less thanabout 50%, or not less than about 60%, or not less than about 70%.

Optical diffuser layer 1220 can be any optical diffuser that is asubstantially forward scattering optical diffuser. For example, in somecases, optical diffuser layer 1220 can be a semi-specular partialreflector that reflects a portion of an incident light and transmitsanother portion of the incident light, where each of the transmitted andreflected portions includes a specular portion and a diffuse portion. Insuch cases, a portion of light reflected by layer 1220 is specularlyreflected and another portion of light reflected by layer 1220 isdiffusely reflected. Similarly, in such cases, a portion of lighttransmitted by layer 1220 is specularly transmitted and another portionof light transmitted by layer 1220 is diffusely transmitted. In somecases, substantially forward scattering optical diffuser layer 1220 canbe or include a substantially forward scattering surface diffuser or asubstantially forward scattering volume diffuser or a substantiallyforward scattering diffuser that is a combination of a surface diffuserand a volume diffuser. In the exemplary display system 1200, opticaldiffuser layer 1220 includes a scattering layer 1224 disposed on anoptically clear substrate 1222.

Light incident on optical diffuser 1220 is scattered substantially inforward directions. For example, a light ray 1270 incident on theoptical diffuser is substantially scattered in the forward direction asa first transmitted light ray 1274 and a first reflected light ray 1272.Light ray 1274 is subsequently totally internally reflected at interface1242 as second reflected light ray 1276 which is substantially scatteredby scattering layer 1224 in a forward direction as a second transmittedlight ray 1278 propagating inside the optical cavity. A substantiallyforward scattering optical diffuser layer 1220 can provide efficientmixing of light emitted by lamps 1201 and 1202 resulting in light source100 uniformly illuminating liquid crystal panel 1280. In some cases,lamps 1201 and 1202 are more optically absorptive than other components,such as, for example, the various specular reflectors, in the opticalcavity. In such cases, a substantially forward light scattering opticaldiffuser 1220 scatters light that is emitted by the lamps substantiallyin the forward directions and away from the lamps, which can result inlight source 100 emitting brighter light.

In some cases and specially for light rays propagating at large anglesrelative to the y-direction, some of the layers disposed on top ofoptical film 1240, such as reflective polarizer layer 1250, substrate1260, and/or a liquid crystal panel 1280 that includes one or more lightabsorbing polarizers, can be more optically absorptive than optical film1240. In such cases, optical film 1240 is advantageously positioned inbetween the more light absorbing layers and optical cavity 1215 toprevent or reduce optical loss by totally internally reflecting lightthat would otherwise be absorbed in the layers above the optical film.

In some cases, optical diffuser 1220 is sufficiently optically diffusiveso as to substantially hide at least some detailed features and/orcomponents, such as lamps 1201 and 1202, in optically reflective cavity1215 from a viewer 1295 that views display system 1200 from above liquidcrystal panel 1280, specially from larger viewing angles. In some cases,optical diffuser 1220 is sufficiently optically diffusive so as toeliminate or substantially reduce the hall of mirrors effect that canoccur and be visible to viewer 1295 when, for example, multiple specularreflections between specular reflectors 1210A-1210D and 1212 create arepeating image pattern. In some cases, optical diffuser 1220 issufficiently optically diffusive to assist in homogenizing light withinoptical cavity 1215 so that light with a substantially uniform intensitycan be delivered to liquid crystal panel 1280.

In the exemplary display system 1200, optical diffuser 1220 is a surfacediffuser meaning that a thin scattering layer 1224 is disposed on anoptically clear non-diffusive substrate 1222. The scattering layer can,for example, be a plurality of beads disposed on substrate 1222, wherethe beads can, for example, be dispersed in a host binder. As anotherexample, scattering layer 1224 can be a surface structure formed in thebottom surface of substrate 1222. In some cases, optical diffuser 1220can be a substantially forward scattering volume diffuser. In general,optical diffuser 1220 can be any type optical diffuser or scatterer thatis substantially forward scattering.

In some cases, side light reflectors 1210A-1210D and back reflector 1212are substantially specular reflectors. For example, in such cases, theratio of the specular reflectance to diffuse reflectance of asubstantially specular reflector is at least about 100, or at leastabout 200, or at least about 300, or at least about 400, or at leastabout 500. In such cases, the diffuse reflectance of the substantiallyspecular reflector is not more than about 2%, or not more than about1.5%, or not more than about 1%, or not more than about 0.5%.

In some cases, at least one of side light reflectors 1210A-1210D andback reflector back reflector 1212 can be a semi-specular reflectormeaning that a portion of an incident light is specularly reflected andanother portion of the incident light is diffusely reflected. In suchcases, the diffusely reflected portion is scattered substantially in theforward direction. For example, in some cases, back reflector 1212 canbe a semi-specular reflector. As another example, in some cases, one ormore of side reflectors 1210A-1210D can be a semi-specular lightreflectors.

Specular reflectors 1210A-1210D and 1212 can be any type specularreflectors that may be desirable and/or practical in an application. Forexample, the reflectors can be aluminized films, silver coated films, ormultilayer polymeric reflective films, such as enhanced specularreflector (ESR) films available from 3M Company, St. Paul, Minn. The ESRfilms have a reflectance of at least about 99% in the wavelength rangefrom about 400 nm to about 1000 nm at normal incidence.

Reflective polarizer layer 1250 substantially reflects light that has afirst polarization state and substantially transmits light that has asecond polarization state, where the two polarization states aremutually orthogonal. In some cases, reflective polarizer 1250substantially reflects light having a first linear polarization state(for example, along the x-direction) and substantially transmits lighthaving a second linear polarization state (for example, along thez-direction).

Any suitable type of reflective polarizer may be used for reflectivepolarizer layer 1250 such as, for example, a multilayer optical film(MOF) reflective polarizer, a diffusely reflective polarizing film(DRPF), a wire grid reflective polarizer, or a cholesteric reflectivepolarizer. In some cases, reflective polarizer layer 1250 can be orinclude a fiber polarizer. In such cases, the reflective polarizerincludes a plurality of substantially parallel fibers that form one ormore layers of fibers embedded within a binder with at least one of thebinder and the fibers including a birefringent material. Thesubstantially parallel fibers define a transmission axis and areflection axis. The fiber polarizer substantially transmits incidentlight that is polarized parallel to the transmission axis andsubstantially reflects incident light that is polarized parallel to thereflection axis. Examples of fiber polarizers are described in, forexample, U.S. Pat. Nos. 7,599,592 and 7,526,164, the entireties of whichare incorporated herein by reference.

In some cases, reflective polarizer layer 1250 can be a partiallyreflecting layer that has an intermediate on-axis average reflectance inthe pass state. For example, the partially reflecting layer can have anon-axis average reflectance of at least about 90% for visible lightpolarized in a first plane, such as the xy-plane, and an on-axis averagereflectance in a range from about 25% to about 90% for visible lightpolarized in a second plane, such as the xz-plane, perpendicular to thefirst plane.

In some cases, reflective polarizer layer 1250 can be an extended bandreflective polarizer that is capable of polarizing light at smallerincident angles and substantially reflecting one polarization state, ortwo mutually perpendicular polarization states, at larger incidentangles as described in U.S. Patent Application Ser. No. 61/254,691titled “Immersed Reflective Polarizer with High Off-Axis Reflectivity”,and U.S. Patent Application Ser. No. 61/254,692 “Immersed ReflectivePolarizer With Angular Confinement in Selected Planes of Incidence”,both filed on even date herewith and the disclosures of which areincorporated herein in their entireties by reference.

In some cases, reflective polarizer layer 1250 can be a diffusereflective polarizer substantially transmitting one polarization stateand substantially diffusely reflecting an orthogonal polarization state.Diffuse reflective polarizer films typically include a disperse phase ofpolymeric particles disposed within a continuous birefringent matrix.The film is oriented, typically by stretching, in one or more directionsto develop the birefrengence. Examples of diffuse reflective polarizersare described in, for example, U.S. Pat. Nos. 6,999,233 and 6,987,612the disclosures of which are incorporated herein in their entireties byreference.

Substrate 1260 is optically transparent and is primarily designed toprovide support to and strengthen optical stack 1290. Substrate 1260 canbe rigid or flexible. Exemplary materials for the substrate includeglass and polymers such as polyethylene terapthalate (PET),polycarbonates, and acrylics.

In some cases, first and second optical adhesive layers 1230 and 1235are primarily designed to bond the layer on one side of the adhesives tothe layer on the other side of the adhesives. In such cases, the primarypurpose of first optical adhesive layer 1230 is to laminate optical film1240 to substantially forward scattering optical diffuser layer 1220 andthe primary purpose of second optical adhesive layer 1235 is to laminatereflective polarizer layer 1250 to support substrate 1260. In suchcases, the optical adhesive layers can have a high specular opticaltransmittance. For example, in such cases, the specular opticaltransmittance of each of the adhesive layers is not less than about 60%,or not less than about 70%, or not less than about 80%, or not less thanabout 90%.

In some cases, one or both of adhesive layers 1230 and 1235 may beabsent in display system 1200. For example, in some cases, displaysystem 1200 may not include first optical adhesive layer 1230. In suchcases, optically diffuser layer 1220 can be coated directly on opticalfilm 1240. In some cases, optical film 1240 is coated on reflectivepolarizer layer 1250. In some cases, optical film 1240 is laminated toreflective polarizer layer 1250 via an adhesive layer not shownexpressly in FIG. 1.

In some cases, optical adhesive layers 1230 and/or 1235 can be opticallydiffusive. For example, in such cases, the optical haze of an opticallydiffusive adhesive layer can be at least about 5%, or at least about10%, or at least about 15%, or at least about 20%. In some cases, thediffuse reflectance of an optically diffusive adhesive layer can be atleast about 5%, or at least about 10%, or at least about 15%, or atleast about 20%. In such cases, the adhesive layer can be opticallydiffusive by, for example, including a plurality of particles dispersedin an optical adhesive binder where the particles and the opticaladhesive binder have different indices of refraction. The mismatchbetween the two indices of refraction can result in light scattering.

Optical adhesive layers 1230 and 1235 can be or include any opticaladhesive that may be desirable and/or available in an application.Exemplary optical adhesives include pressure sensitive adhesives (PSAs),heat-sensitive adhesives, solvent-volatile adhesives, and UV-curableadhesives such as UV-curable optical adhesives available from NorlandProducts, Inc. Exemplary PSAs include those based on natural rubbers,synthetic rubbers, styrene block copolymers, (meth)acrylic blockcopolymers, polyvinyl ethers, polyolefins, and poly(meth)acrylates. Asused herein, (meth)acrylic (or acrylate) refers to both acrylic andmethacrylic species. Other exemplary PSAs include (meth)acrylates,rubbers, thermoplastic elastomers, silicones, urethanes, andcombinations thereof. In some cases, the PSA is based on a (meth)acrylicPSA or at least one poly(meth)acrylate. Exemplary silicone PSAs includea polymer or gum and an optional tackifying resin. Other exemplarysilicone PSAs include a polydiorganosiloxane polyoxamide and an optionaltackifier.

In some cases, one or both of optical adhesive layers 1230 and 1235 canbe a removable adhesive such as those described in, for example, U.S.Pat. Nos. 3,691,140; 4,166,152; 4,968,562; 4,994,322; 5,296,277;5,362,516, the disclosures of which are incorporated herein in theirentireties by reference. The phrase “removable adhesive” for adhering afilm to a substrate means an adhesive that affords convenient, manualremoval of the film from the substrate without damaging the substrate orexhibiting excessive adhesive transfer from the film to the substrate.

In some cases, one or both of optical adhesive layers 1230 and 1235 canbe a reusable and/or repositionable adhesive such as those described in,for example, U.S. Pat. No. 6,197,397; U.S. Patent Publication No.2007/0000606; and PCT Publication No. WO 00/56556, the disclosures ofwhich are incorporated herein in their entireties by reference. Thephrases “reusable adhesive” or “repositionable adhesive” for adhering afilm to a substrate mean an adhesive that (a) affords a temporary,secure attachment of the film to the substrate while affordingconvenient, manual removal of the film from the substrate withoutdamaging the substrate or exhibiting excessive adhesive transfer fromthe film to the substrate, and (b) then affords subsequent reuse of thefilm on, for example, another substrate.

Substantial portions of each two neighboring major surfaces in opticalstack 1290 are in physical contact with each other. For example,substantial portions of neighboring major surfaces 1241 and 1251 ofrespective neighboring layers 1240 and 1250 in optical stack 1290 are inphysical contact with each other. For example, at least 50%, or at least60%, or at least 70%, or at least 80%, or at least 90%, or at least 95%of the two neighboring major surfaces are in physical contact with eachother. For example, in some cases, optical film 1240 is coated directlyon reflective polarizer layer 1250.

In general, substantial portions of neighboring major surfaces (majorsurfaces that face each other or are adjacent to each other) of each twoneighboring layers in optical stack 1290 are in physical contact witheach other. For example, in some cases, there may be one or moreadditional layers, such as a support layer or an adhesive layer,disposed between reflective polarizer layer 1250 and optical film 1240.In such cases, substantial portions of neighboring major surfaces ofeach two neighboring layers in optical stack 1290 are in physicalcontact with each other. In such cases, at least 50%, or at least 60%,or at least 70%, or at least 80%, or at least 90%, or at least 95% ofthe neighboring major surfaces of each two neighboring layers in theoptical stack are in physical contact with each other.

In the exemplary optical stack 1290, optical film 1240 physicallycontacts reflective polarizer layer 1250. For example, optical film 1240can be coated directly on bottom surface 1251 of reflective polarizerlayer 1250. In some cases, one or more layers can be disposed betweenthe two layers. For example, FIG. 4 is a schematic side-view of anoptical stack 1290 that includes an optical adhesive layer 410 and asubstrate layer 420 disposed between optical film 1240 and reflectivepolarizer layer 1250 where substrate layer 420 can be a support layerfor optical film 1240 and optical adhesive layer can be a bonding layerfor bonding or laminating the optical film to the reflective polarizerlayer.

Referring back to FIG. 1, liquid crystal panel 1280 includes, notexpressly shown in FIG. 1, a layer of liquid crystal disposed betweentwo panel plates, an upper light absorbing polarizer layer disposedabove the liquid crystal layer and a lower absorbing polarizer disposedbelow the liquid crystal layer. The upper and lower light absorbingpolarizers and the liquid crystal layer, in combination, control thetransmission of light from reflective polarizer layer 1250 throughliquid crystal panel 1280 to viewer 1295.

Lamps 1201 and 1202 can be any type of lamp that may be desirable and/orpractical in an application. For example, the lamps can be extendeddiffuse lamps such as cold cathode fluorescent lamps (CCFLs), smallerarea solid state lamps such as light emitting diodes (LEDs), or lasers.In some cases, one or more of lamps 1201 and 1202 can include differenttype lamps. For example, lamps 1201 can include a combination of LEDsand CCFLs. In some cases, the lamps can emit light in differentwavelength regions. For example, lamps 1201 can include a first lampemitting red light, a second lamp emitting green light, and a third lampemitting blue light.

In the exemplary display system 1200, substantially forward scatteringoptical diffuser layer 1220 is placed at output port 1204C of opticallyreflective cavity 1215. In some cases, such as when the primary functionof the diffuser layer is to assist in mixing and homogenizing lightinside optical cavity 1215, diffuser layer 1220 can be placed, withproper orientation, at other locations within the optical cavity. Forexample, FIG. 3 is a schematic side-view of a display system 1300 thatis similar to display system 1200 except that optical diffuser layer1220 in display system 1300 is placed within the interior of opticalcavity 1215 in the xz-plane so that the forward scattering properties ofthe optical diffuser can assist in efficient mixing of light emitted bythe lamps. As another example, optical diffuser 1220 and/or scatteringlayer 1224 can be disposed on one or more reflectors in the opticalcavity. For example, FIG. 3 illustrates a scattering layer 1324, similarto scattering layer 1224, disposed on back reflector 1212. In theexemplary display system 1300, optical film 1240 is placed at outputport 1204C of optical cavity 1215.

Low index properties of optical film 1240 and the substantially forwardscattering properties of optical diffuser layer 1220 can advantageouslyprovide for a compact and thin optically reflective cavity 1215 andimproved light mixing. For example, in some cases, the maximum lateraldimension, such as the size of the diagonal or length of opticallyreflective cavity 1215 is substantially greater than the maximumthickness of the reflective cavity. For example, in such cases, theratio of the maximum lateral (in the xz-plane) dimension of opticallyreflective cavity 1215 to the maximum thickness (along the y-direction)of the optically reflective cavity is not less than about 20, or notless than about 40, or not less than about 60, or not less than about80, or not less than about 100. In some cases, the maximum thickness hiof optically reflective cavity 1215 in FIG. 1 is in a range from about 2mm to about 50 mm, or from about 5 mm to about 40 mm, or from about 7 mmto about 30 mm, or from about 10 mm to about 20 mm.

In some cases, a display system, such as an LCD system, can incorporatea backlight for uniform illumination of a liquid crystal panel where thebacklight includes light source 100 with no additional layers. In somecases, the backlight can include light source 100 and one or moreadditional layers, such as one or more additional light managementlayers or films. Examples of light management films include reflectivepolarizers, light redirecting films such as a brightness enhancementfilms (for example, BEF available from 3M Company, Saint Paul Minn.),turning films (for example, an inverted BEF), optical diffusers, or anyother light management layer that may be desirable in an application.

In the exemplary display system 1200, layer 1250 is a reflectivepolarizer. In some cases, layer 1250 can be a non-polarizing partiallyreflective partially transmissive layer transmitting a portion of anincident light as an unpolarized transmitted light and reflecting aportion, such as least 30% or at least 40% or at least 50%, of theincident light as an unpolarized reflected light. In some cases, anon-polarizing partially reflective partially transmissive layer canalso absorb a portion of the incident light. The partially reflectivepartially transmissive layer can be a multilayer optical film, or ametal, such as Al or Ag or Ni, coated film. In some cases, anon-polarizing partially reflective partially transmissive layer can beor include foams or microreplictated structures.

In some cases, optically reflective hollow cavity 1215 can includeextraction features to assist in extracting light from the cavity. Forexample, FIG. 12 is a schematic side-view of a display system 1232 thatis similar to display system 1200 except that optically reflectivecavity 1215 in FIG. 12 includes a plurality of extraction features 1231disposed on back reflector 1212. Extraction features 1231 assist inextracting light from the reflective cavity from output port 1204C.Extraction features 1231 can be any extraction features capable ofextracting or assisting in extracting light from the cavity. Forexample, the extraction features can be features that are for example,printed, cast or stamped, on the back reflector. In some cases,extraction features 1231 can be arranged to enhance or increase thebrightness along a desired, such as the on-axis, direction.

Optically reflective cavity 1215 can include one or more opticalelements not expressly shown in FIG. 1. For example, FIG. 18 is aschematic side-view of a display system 1900, where optically reflectivehollow cavity 1215 includes an optical element 1910 receiving light 1920emitted by lamps 1202. Optical element 1910 can be or include an opticalfilter reflecting and/or absorbing a portion, such as a UV portion, ofincident light 1920. As another example, optical element 1910 can be orinclude an asymmetric, such as a one-dimensional, optical diffuser forspreading emitted light 1920 more along a particular direction, such asthe z-direction, and less along other directions. As another example,optical element 1910 can be or include a wavelength converter forconverting, such as down converting, light 1920 to a different, such aslonger, wavelength light 1930. As yet another example, optical element1910 can be or include a light collimator receiving a less collimatedlight 1920 and transmitting a more collimated light 1930.

In some cases, light emitted by a lamp in optically reflective cavity1215 can be delivered to the optical cavity via one or more hollow orsolid light guides, such as for example, one or more optical fibers. Forexample, in FIG. 18, light from a lamp 1940 is delivered to opticallyreflective cavity 1215 at input port 1204A via optical fibers 1950.

In the exemplary display system 1200 in FIG. 1, reflective polarizer1250 and back reflector 1212 are planar and non-parallel relative toeach other. In general, the orientation of the reflective polarizerlayer and the back reflector relative to each other can be anyorientation that may be desirable in an application. For example, insome cases, the reflective polarizer layer can be parallel to the backreflector. In some cases, the reflective polarizer layer can benon-parallel to the back reflector. In some cases, one or both of thetwo layers can be planar or non-planar, such as curved.

In some cases, optical stack 1290 can be bonded to liquid crystal paneland can have fewer layers. For example, FIG. 19 is a schematic side-viewof an optical construction 1900 that includes liquid crystal panel 1280disposed on an optical stack 1990. Optical stack 1990 can, in somecases, replace optical stack 1290 in FIG. 1 and is laminated or bondedto liquid crystal panel 1280 via optical adhesive layer 1235. Opticalstack 1990 includes optical diffuser layer 1220 disposed at output port1204C, optical film 1240 disposed on the optical diffuser layer,reflective polarizer layer 1250 disposed on the optical film, andoptical adhesive layer 1235 disposed on the reflective polarizer layer.In some cases, there can be one or more layers between any twoneighboring layers in optical stack 1990 or optical construction 1900.

In some cases, a disclosed optical stack can include an optical filmdisposed between a liquid crystal panel and a reflective polarizerlayer. For example, FIG. 20 is a schematic side-view of an opticalconstruction 2000 that includes liquid crystal panel 1280 disposed on anoptical stack 2090. Optical stack 2090 can, in some cases, replaceoptical stack 1290 in FIG. 1 and is laminated or bonded to liquidcrystal panel 1280 via optical adhesive layer 1235. Optical stack 2090includes optical diffuser layer 1220 disposed at output port 1204C,reflective polarizer layer 1250 disposed on the optical diffuser layer,optical film 1240 disposed, for example coated, on the reflectivepolarizer layer, and optical adhesive layer 1235 disposed on the opticalfilm. In some cases, there can be one or more layers between any twoneighboring layers in optical stack 2090 or optical construction 2000.

FIG. 5 is a schematic side-view of a display system 1201 that includesliquid crystal panel 1280 disposed on a light source 500. The lightsource includes an optical stack 1291 that receives light from opticallyreflective cavity 1215. Optical stack 1291 includes a substantiallyforward scattering optical film 1285 that is disposed at output port1204C of optically reflective cavity 1215, reflective polarizer layer1250 disposed on the optical film, optical adhesive layer 1330 disposedon the reflective polarizer layer, and substrate 1260 disposed on theoptical adhesive layer.

A first major surface 1251 of reflective polarizer layer 1250 facesoptical film 1285. Optical film 1285 includes a first major surface 1286that faces optically reflective cavity 1215 and a second major surface1287 that faces the reflective polarizer layer and neighbors majorsurface 1251. Substantial portions of neighboring major surfaces 1251and 1287 of the two neighboring layers 1250 and 1285 in optical stack1291 are in physical contact with each other. For example, at least 50%,or at least 60%, or at least 70%, or at least 80%, or at least 90%, orat least 95% of the two neighboring major surfaces are in physicalcontact with each other.

In general, substantial portions of neighboring major surfaces (majorsurfaces that face each other or are adjacent to each other) of each twoneighboring layers in optical stack 1291 are in physical contact witheach other. For example, in some cases, there may be one or moreadditional layers, such as an adhesive layer and/or a substrate layernot expressly shown in FIG. 5, disposed in between reflective polarizerlayer 1250 and optical film 1285. In such cases, substantial portions ofneighboring major surfaces of each two neighboring layers in opticalstack 1291 are in physical contact with each other. In such cases, atleast 50%, or at least 60%, or at least 70%, or at least 80%, or atleast 90%, or at least 95% of the neighboring major surfaces of each twoneighboring layers in the optical construction are in physical contactwith each other.

Substantially forward scattering optical film 1285 includes a pluralityof voids, such as interconnected voids, dispersed in a binder. In somecases, optical film 1285 also includes a plurality of particlesdispersed in the binder and/or the optical film. Optical film 1285 canbe any optical film disclosed herein that includes voids and issubstantially forward scattering. In some cases, optical film 1285 has alow optical haze and diffuse reflectance.

In some cases, optical film 1285 has a high optical haze. In such cases,the optical haze of the optical film is not less than about 30%, or notless than about 40%, or not less than about 50%, or not less than about60%, or not less than about 70%, or not less than about 80%, or not lessthan about 90%. In some cases, optical film 1285 has a high diffuseoptical reflectance. In such cases, the diffuse optical reflectance ofthe optical film is not less than about 30%, or not less than about 40%,or not less than about 50%, or not less than about 60%. In some cases,optical film 1285 has a low optical clarity. In such cases, the opticalclarity of the optical film is not greater than about 70%, or notgreater than about 60%, or not greater than about 50%, or not greaterthan about 40%, or not greater than about 30%, or not greater than about20%, or not greater than about 10%.

In some cases, optical film 1285 has a high optical haze and manifestssome-low-index like properties. For example, in such cases, optical film1285 can support TIR or enhance internal reflection. For example, alight ray 1289 that is incident on the interface between the opticalfilm and reflective polarizer layer 1250 with an incident angle θ, canunder go TIR because the optical film has a low effective index. In somecases, it may not be possible to assign an effective index to theoptical film because of, for example, high optical haze, but the opticalfilm can still enhance internal reflection meaning that the reflectionis greater than what the binder of the optical film would produce.

An advantage of optical stack 1291 is that optical film 1285 has highoptical haze and can substantially scatter light while, at the sametime, it can manifest some low-index properties. For example, opticalstack 1291 can have an appreciable optical gain. For example, opticalgain of optical stack 1291 can be at least about 1.1, or at least about1.2, or at least about 1.2, or at least about 1.25, or at least about1.3, or at least about 1.35, or at least about 1.4, or at least about1.45, or at least about 1.5. As used herein, “gain” or “optical gain” ofan optical stack is defined as the ratio of the axial output luminanceof an optical or display system with the optical stack to the axialoutput luminance of the same optical or display system without theoptical stack.

In the exemplary display system 1201, major surface 1286 of optical film1285 is structured and can scatter light. In general, major surface 1286can have any properties that may be desirable in an application. Forexample, in some cases, major surface 1285 can be smooth.

Optical adhesive layer 1330 bonds reflective polarizer layer 1250 tosupport substrate 1260. Optical adhesive layer can be similar to anyoptical adhesive layer disclosed herein, such as optical adhesive layers1230 and 1235.

The display systems, light sources, and optically reflective cavitiesdisclosed herein can have any shape and configuration that may bedesirable in an application. For example, in some cases, a discloseddisplay system such as display system 1200, a disclosed light sourcesuch as light source 100, and/or a disclosed optically reflective cavitysuch as cavity 1215, can be planar or curved.

In the exemplary display system 1201, output port 1204C is substantiallythe same size as back reflector 1212. In general, output port 1204C canhave any size and/or shape that may be desirable in an application. Forexample, FIG. 14 is a schematic side-view of a display system 1500 thatis similar to display system 1201 and includes optical stack 1291disposed at an output port 1504C of an optically reflective cavity 1515,where output port 1504C is smaller than specular back reflector 1212.Optical cavity 1515 includes top specular reflectors 1520A and 1520B. Insome cases, at least one of top reflectors 1520A and 1520B can be asemi-specular reflector.

Referring back to FIG. 5, in some cases, optical stack 1291 can havefewer layers or one or more additional layers. For example, FIG. 15 is aschematic side-view of an optical stack 1690 that can, for example,replace optical stack 1291. Optical stack 1690 is bonded to liquidcrystal panel 1280 via an optical adhesive layer 1605. Optical stack1690 includes optical film 1285, reflective polarizer layer 1250disposed on optical film 1285, a structured light redirecting film 1620disposed on the reflective polarizer layer, an optical film 1610disposed on and planarizing the light redirecting film, and opticaladhesive layer 1605 disposed on optical film 1610. Light redirectingfilm 1620 includes a structured top surface 1621 that includes aplurality of linear prisms 1622 extending along the z-direction. Opticalfilm 1610 planarizes structured surface 1621 and optically couples toliquid crystal panel 1280 via optical adhesive layer 1605. Optical film1610 can be any optical film disclosed herein. For example, in somecases, optical film 1610 includes voids dispersed in a binder and has aneffective index of refraction that is not greater than about 1.3, or notgreater than about 1.25, or not greater than about 1.2, or not greaterthan about 1.15, or not greater than about 1.1, or not greater thanabout 1.05. In some cases, the optical haze of optical film 1610 is notgreater than about 5%, or not greater than about 4%, or not greater thanabout 3%, or not greater than about 2%, or not greater than about 1%, ornot greater than about 0.5%.

Substantial portions of neighboring major surfaces (major surfaces thatface each other or are adjacent to each other) of each two neighboringlayers in optical stack 1690 are in physical contact with each other. Insome cases, there may be one or more additional layers, such as anadhesive layer and/or a substrate layer not expressly shown in FIG. 15,disposed in between, for example, reflective polarizer layer 1250 andoptical film 1285. In such cases, substantial portions of neighboringmajor surfaces of each two neighboring layers in optical stack 1690 arein physical contact with each other. In such cases, at least 50%, or atleast 60%, or at least 70%, or at least 80%, or at least 90%, or atleast 95% of the neighboring major surfaces of each two neighboringlayers in the optical stack are in physical contact with each other.

Light redirecting film 1620 can be any film that includes structurescapable of redirecting light. Examples of light redirecting filmsinclude brightness enhancement films (for example, BEF available from 3MCompany, Saint Paul Minn.) and turning films (for example, an invertedBEF).

As yet another example, FIG. 21 is a schematic side-view of an opticalstack 2100 that can replace optical stack 1291 or any other opticalstack disclosed herein. Optical stack 2100 includes a first lightredirecting film 2110, a first optical film 2120 disposed on andplanarizing the first light redirecting film, a first optical adhesivelayer 2130 disposed on the first optical film, an optical diffuser layer2140 disposed on the first optical adhesive layer, a second optical film2150 disposed on and planarizing the optical diffuser layer, a secondoptical adhesive layer 2160 disposed on the second optical film, asecond light redirecting layer 2170 disposed on the second opticaladhesive layer, and a third optical film 2180 disposed on andplanarizing the second light redirecting layer.

First and second light redirecting films 2110 and 2170 can be any lightredirecting films that may be desirable in an application. For example,in some cases, light redirecting films 2110 and 2170 can be similar tolight redirecting film 1620. In some cases, light redirecting films 2110and 2170 include linear prismatic films with the linear prisms in onelight redirecting film being oriented along a first direction and thelinear prisms in the other light redirecting film being oriented along asecond direction orthogonal to the first direction. For example, in somecases, the linear prisms in light redirecting film 2110 can extend, orbe oriented, along the x-direction and the linear prisms in lightredirecting film 2170 can extend, or be oriented, along the z-direction.In such cases and with optical stack 2100 disposed at, for example,output port 1204C, the prisms in first or lower light redirecting film2110 can efficiently totally internally reflect substantial portions oflight that is emitted by light sources 1201 and 1202 and travel alongthe general x-direction.

Optical films 2120, 2150 and 2180 can be nay optical films disclosedherein, such as optical films 1240 and 1285. For example, in some cases,the optical films include voids, such as interconnected voids, dispersedin a binder and have effective indices of refraction that are notgreater than about 1.3, or not greater than about 1.25, or not greaterthan about 1.2, or not greater than about 1.15, or not greater thanabout 1.1, or not greater than about 1.05. In some cases, the opticalhaze of the optical films is not greater than about 5%, or not greaterthan about 4%, or not greater than about 3%, or not greater than about2%, or not greater than about 1%, or not greater than about 0.5%.

Optical adhesive layers 2130 and 2160 can be any optical adhesive layerdisclosed herein, such as optical adhesive layers 1230, 1235 and 1330.Optical diffuser layer 2140 can be similar to any optical diffuser layerdisclosed herein, such as optical diffuser layer 1220. In some cases,optical diffuser layer includes a plurality of beads dispersed in abinder, where the beads form a top structured surface. In some cases,the index of the binder and the beads are substantially the same. Insuch cases, optical diffuser layer 2140 is substantially a surfacediffuser and scatters no, or very little, light at a volume diffuser. Insuch cases, optical diffuser layer 2140 can enhance the optical gain ofoptical stack 2100.

Substantial portions of neighboring major surfaces (major surfaces thatface each other or are adjacent to each other) of each two neighboringlayers in optical stack 2100 are in physical contact with each other. Insome cases, there may be one or more additional layers, such as anadhesive layer and/or a substrate layer not expressly shown in FIG. 21,disposed in between, for example, first optical film 2120 and firstlight redirecting film 2110. In such cases, substantial portions ofneighboring major surfaces of each two neighboring layers in opticalstack 2100 are in physical contact with each other. In such cases, atleast 50%, or at least 60%, or at least 70%, or at least 80%, or atleast 90%, or at least 95% of the neighboring major surfaces of each twoneighboring layers in the optical stack are in physical contact witheach other.

As another example, FIG. 16 is a schematic side-view of an optical stack1790 that can replace optical stack 1291. Optical stack 1790 is bondedto liquid crystal panel 1280 via an optical adhesive layer 1710 andincludes optical film 1285, an optical film 1730 disposed on opticalfilm 1285, light redirecting film 1620 disposed on optical film 1730,optical film 1610 disposed on and planarizing light redirecting film1620, an optical adhesive layer 1720 disposed on optical film 1610,reflective polarizer layer 1250 disposed on optical adhesive layer 1720,and optical adhesive layer 1710 disposed on reflective polarizer layer1250.

Optical film 1730 can be any optical film disclosed herein. For example,in some cases, optical film 1730 includes voids dispersed in a binderand has an effective index of refraction that is not greater than about1.3, or not greater than about 1.25, or not greater than about 1.2, ornot greater than about 1.15, or not greater than about 1.1, or notgreater than about 1.05. In some cases, the optical haze of optical film1730 is not greater than about 5%, or not greater than about 4%, or notgreater than about 3%, or not greater than about 2%, or not greater thanabout 1%, or not greater than about 0.5%.

Substantial portions of neighboring major surfaces (major surfaces thatface each other or are adjacent to each other) of each two neighboringlayers in optical stack 1790 are in physical contact with each other. Insome cases, there may be one or more additional layers, such as anadhesive layer and/or a substrate layer not expressly shown in FIG. 16,disposed in between, for example, light redirecting film 1620 andoptical film 1730. In such cases, substantial portions of neighboringmajor surfaces of each two neighboring layers in optical stack 1790 arein physical contact with each other. In such cases, at least 50%, or atleast 60%, or at least 70%, or at least 80%, or at least 90%, or atleast 95% of the neighboring major surfaces of each two neighboringlayers in the optical stack are in physical contact with each other.

FIG. 13 is a schematic side-view of a display system 1400 that includesa light source 1401 providing illumination to a first liquid crystalpanel 1440A viewable by a viewer 1450A and a second liquid crystal panel1440B viewable by a second viewer 1450B. Light source 1401 includes anoptically reflective cavity 1405 that includes an input port 1403receiving light from a lamp 1402, a first output port 1404A fortransmitting light towards and illuminating first liquid crystal panel1440A, and a second output port 1404B for transmitting light towards andilluminating second liquid crystal panel 1440B. Light source 1401 alsoincludes a first optical stack 1490A disposed at first output port 1404Aand a second optical stack 1490B disposed at second output port 1404B.

Optical reflective cavity includes respective first and second specularside reflectors 1410A and 1410B and specular end reflector 1410C. Eachof optical stacks 1490A and 1490B includes a reflective polarizer layerdisposed on an optical film. In particular, first optical stack 1490Aincludes a first optical film 1420A disposed at first output port 1404Aand a first reflective polarizer layer 1430A disposed on first opticalfilm 1420A, and second optical stack 1490B includes a second opticalfilm 1420B disposed at second output port 1404B and a second reflectivepolarizer layer 1430B disposed on second optical film 1420A.

In some cases, at least one of first and second optical films 1420A and1420B is a substantially forward scattering optical film. In such cases,the optical film has a transport ratio that is not less than about 0.2,or not less than about 0.3, or not less than about 0.4, or not less thanabout 0.5, or not less than about 0.6, or not less than about 0.8. Eachof optical films 1420A and 1420B transmits a portion of an incidentlight and reflects another portion of the incident light. In some cases,the optical reflectance of at least one of optical films 1420A and 1420Bis at least 40%, or at least 50%, or at least 60%, or at least 70%, orat least 80%, or at least 90%. In some cases, the optical transmittanceof at least one of optical films 1420A and 1420B is not greater thanabout 30%, or not greater than about 25%, or not greater than about 20%,or not greater than about 15%, or not greater than about 10%. In somecases, such as when light source 1401 provides uniform illumination, theoptical haze of at least one of substantially forward scattering opticalfilms 1420A and 1420B is not less than about 20%, or not less than about30%, or not less than about 40%, or not less than about 50%, or not lessthan about 60%, or not less than about 70%.

Substantial portions of neighboring major surfaces (major surfaces thatface each other or are adjacent to each other) of each two neighboringlayers in each of optical stacks 1490A and 1490B are in physical contactwith each other. For example, in optical stack 1490A, substantialportions of the major bottom surface of first reflective polarizer layer1430A and the major top surface of first optical film 1420A are inphysical contact with each other. In such cases, at least 50%, or atleast 60%, or at least 70%, or at least 80%, or at least 90%, or atleast 95% of each two neighboring major surfaces are in physical contactwith each other.

In general, substantial portions of neighboring major surfaces (majorsurfaces that face each other or are adjacent to each other) of each twoneighboring layers in each of optical stacks 1490A and 1490B are inphysical contact with each other. For example, in some cases, there maybe one or more additional layers, such as an adhesive layer and/or asubstrate layer not expressly shown in FIG. 13, disposed in between, forexample, reflective polarizer layer 1430A and optical film 1420A. Insuch cases, substantial portions of neighboring major surfaces of eachtwo neighboring layers in optical stack 1490A are in physical contactwith each other. In such cases, at least 50%, or at least 60%, or atleast 70%, or at least 80%, or at least 90%, or at least 95% of theneighboring major surfaces of each two neighboring layers in the opticalstack are in physical contact with each other.

Each of optical films 1420A and 1420B includes a plurality of voids,such as interconnected voids, dispersed in a binder. In some cases, atleast one of optical films 1420A and 1420B also includes a plurality ofparticles dispersed in the binder and/or the optical film. Optical films1420A and 1420B can be any optical film disclosed herein that includesvoids and is substantially forward scattering. In some cases, at leastone of optical films 1420A and 1420B has a low optical haze and diffusereflectance.

In some cases, at least one of optical films 1420A and 1420B has a highoptical haze. In such cases, the optical haze of the optical film is notless than about 30%, or not less than about 40%, or not less than about50%, or not less than about 60%, or not less than about 70%, or not lessthan about 80%, or not less than about 90%. In some cases, at least oneof optical films 1420A and 1420B has a high diffuse optical reflectance.In such cases, the diffuse optical reflectance of the optical film isnot less than about 30%, or not less than about 40%, or not less thanabout 50%, or not less than about 60%. In some cases, at least one ofoptical films 1420A and 1420B has a low optical clarity. In such cases,the optical clarity of the optical film is not greater than about 70%,or not greater than about 60%, or not greater than about 50%, or notgreater than about 40%, or not greater than about 30%, or not greaterthan about 20%, or not greater than about 10%.

In some cases, at least one of optical films 1420A and 1420B has a highoptical haze and manifests some-low-index like properties. For example,in such cases, each of first and second optical films 1420A and 1420Bcan support TIR or enhance internal reflection. In some cases, it maynot be possible to assign an effective index to the optical film becauseof, for example, high optical haze, but the film can still enhanceinternal reflection meaning that the reflection is greater than what thebinder of the optical film would produce.

An advantage of optical stacks 1490A and 1490B is that the optical filmscan have high optical haze and can substantially scatter light while, atthe same time, they can manifest some low-index properties. For example,optical stack 1490A can have an appreciable optical gain. For example,optical gain of optical stack 1490A can be at least about 1.1, or atleast about 1.2, or at least about 1.2, or at least about 1.25, or atleast about 1.3, or at least about 1.35, or at least about 1.4, or atleast about 1.45, or at least about 1.5.

In the exemplary display system 1400, major surface 1421A of opticalfilm 1420A is structured and can scatter light, and major surface 1421Bof optical film 1420B is structured and can scatter light. In general,each of major surfaces 1421A and 1421B can have any properties that maybe desirable in an application. For example, in some cases, at least oneof major surfaces 1421A and 1421B can be smooth.

In some cases, first reflective polarizer layer 1430A can be bonded tofirst liquid crystal panel 1440A via, for example, an optical adhesivelayer, and second reflective polarizer layer 1430B can be bonded tosecond liquid crystal panel 1440B also via, for example, an opticaladhesive layer not expressly shown in FIG. 13.

In the exemplary display system 1400, light source 1401 providesillumination to liquid crystal panels 1440A and 1440B for displayingimages and/or information to, for example, viewers 1450A and 1450B. Insome cases, light source 1401 can provide illumination in generallighting applications.

FIG. 17 is a schematic three-dimensional view of a display system 1800for displaying an image and/or data to a viewer 1850. The display systemincludes liquid crystal panel 1280 receiving light from a backlight1810. Backlight 1810 includes a plurality of light sources 1820 thatform an array, such as a regular array. Light sources 1820 can be anylight source disclosed herein, such as light source 100 or 500. In somecases, each light source 1820 can be independently controlled. Forexample, in such cases, the brightness of light emitted by each lightsource can be independently controlled. In some cases, rows or columnsof light source can be independently controlled.

In some cases, backlight 1810 can be actively and locally controlled by,for example, reducing the brightness of a zone of light sources 1820that corresponds to a dark portion of a displayed image. Such activezonal control of light sources 1820 can reduce power consumption andenhance display contrast.

In some cases, backlight 1810 can be a tiled backlight or a tiled lightsource that includes a plurality of light source tiles 1820, where atleast one of the light source tiles includes a light source disclosedherein. In some cases, the light source tiles can be interleaved meaningthat portions of neighboring tiles overlap. In some cases, liquidcrystal panel 1280 can be a monolithic image forming panel or a tiledimage forming panel that includes a plurality of image forming tiles.

The exemplary display system 1800 has a rectangular shape display. Ingeneral, the display size and shape can be any size and shape that maybe desirable in an application. For example, in some cases, the displaycan have a regular shape, such as a round shape or an elliptical shape.As another example, in some cases, the display can have an irregularshape.

Some of the advantages of the disclosed optical films, layers, stacks,and systems are further illustrated by the following examples. Theparticular materials, amounts and dimensions recited in these examples,as well as other conditions and details, should not be construed tounduly limit the present invention.

Example A

A coating solution “A” was made. First, 360 g of Nalco 2327 colloidalsilica (40% wt solid) (available from Nalco Chemical Company, NapervilleIll.) and 300 g of solvent 1-methoxy-2-propanol were mixed togetherunder rapid stirring in a 2-liter three-neck flask that was equippedwith a condenser and a thermometer. Next, 22.15 g of Silquest A-174silane (available from GE Advanced Materials, Wilton Conn.) was added.The mixture was stirred for 10 min. Next, an additional 400 g of1-methoxy-2-propanol was added. The mixture was heated at 85° C. for 6hours using a heating mantle. The resulting solution was allowed to cooldown to room temperature. Next, most of the water and1-methoxy-2-propanol solvents (about 700 g) were removed using a rotaryevaporator under a 60° C. water-bath. The resulting solution was 44% wtA-174 modified 20 nm silica clear dispersed in 1-methoxy-2-propanol.Next, 70.1 g of this solution, 20.5 g of SR 444 (available from SartomerCompany, Exton Pa.), 1.375 g of photoinitiator Irgacure 184 (availablefrom Ciba Specialty Chemicals Company, High Point N.C.), and 80.4 g ofisopropyl alcohol were mixed together by stirring to form a homogenouscoating solution A.

Example B

A coating procedure “B” was developed. First, a coating solution wassyringe-pumped at a rate of 2.7 cc/min into a 20.3 cm wide slot-typecoating die. The slot coating die uniformly distributed a 20.3 cm widecoating onto a substrate moving at 152 cm/min.

Next, the coating was polymerized by passing the coated substratethrough a UV-LED cure chamber that included a quartz window to allowpassage of UV radiation. The UV-LED bank included a rectangular array of352 UV-LEDs, 16 down-web by 22 cross-web (approximately covering a 20.3cm×20.3 cm area). The UV-LEDs were placed on two water-cooled heatsinks. The LEDs (available from Cree, Inc., Durham N.C.) operated at anominal wavelength of 395 nm and were run at 45 Volts at 10 Amps,resulting in a UV-A dose of 0.108 joules per square cm. The UV-LED arraywas powered and fan-cooled by a Lambda GENH 60-12.5-U power supply(available from TDK-Lambda, Neptune N.J.). The UV-LEDs were positionedabove the cure chamber quartz window at a distance of approximately 2.54cm from the substrate. The UV-LED cure chamber was supplied with a flowof nitrogen at a flow rate of 46.7 liters/min (100 cubic feet per hour)resulting in an oxygen concentration of approximately 150 ppm in thecure chamber.

After being polymerized by the UV-LEDs, the solvent in the cured coatingwas removed by transporting the coating to a drying oven operating at150° F. Next, the dried coating was post-cured using a Fusion SystemModel 1300P configured with an H-bulb (available from Fusion UV Systems,Gaithersburg Md.). The UV Fusion chamber was supplied with a flow ofnitrogen that resulted in an oxygen concentration of approximately 50ppm in the chamber.

Example 1

A light source 2800, a side-view of which is schematically shown in FIG.6, was made. Light source 2800 included an optically reflective hollowcavity 2810 and an optical stack 2890 that was placed at an open frontport 2822 of the reflective cavity.

The optical cavity was wedge shaped and included a proximal sidereflector 2824, a distal side reflector 2820, a bottom reflector 2822,and a lamp 2830 housed within a parabolic reflector 2840. The opticalcavity had the following dimensions: l₁=1 mm, l₂=16 mm, l₃=400 mm, l₄=17mm, and l₅=21 mm. Lamp 2830 included 12 white LEDs (available under thename Luxeon Rebel from Philips Lumiled Lighting Company, San Jose,Calif.). The LEDs were placed on heat sinks not expressly shown in FIG.6.

The interiors of the parabolic reflector, the side reflectors and thebottom reflector were lined with ESR mirror films (available as from 3MCompany, St. Paul Minn.) having a 99.5% reflectance in the visible.Optically reflective cavity 2810 had an open output port 2826 fortransmitting light that was emitted by lamp 2830.

Optical stack 2890 included an optical diffuser 2850 coated on areflective polarizer layer 2860. The reflective polarizer was laminatedto a substrate 2880 via an optical adhesive layer 2870. Substrate 2880was a 1.5 mm thick polycarbonate (PC) sheet. Optically clear adhesive2870 was adhesive OCA 8173 (available from 3M Company, St. Paul Minn.).

Reflective polarizer layer 2860 had a pass axis along the x-axis and ablock axis along the y-axis. The average on-axis (along the z-direction)reflectivity of the reflecting polarizer layer for incident lightpolarized along the x-axis (the pass axis) was about 68%, and theaverage on-axis (along the z-direction) reflectivity of the reflectingpolarizer layer for incident light polarized along the y-axis (the blockaxis) was about 99.2%. The reflective polarizer layer was made asdescribed in International Publication No. WO 2008/144656 filed on May19, 2008, the disclosure of which is incorporated in its entirety hereinby reference.

The reflective polarizer layer included 274 alternating microlayers ofbirefringent 90/10 coPEN material and Eastman Neostar Elastomer FN007(available from Eastman Chemical, Kingsport Tenn.). The 274 alternatingmicrolayers were arranged in a sequence of ¼ wave layer pairs, where thethickness gradient of the layers was designed to provide a strongreflection resonance broadly and uniformly across a bandwidth fromapproximately 400 nm to 1050 nm for one polarization axis, and a weakerreflection resonance broadly and uniformly across a bandwidth fromapproximately 400 nm to 900 nm for the orthogonal axis. Two 5 micronthick skin layers of PET-G were disposed on the outside surfaces of thecoherent altering microlayer stack. The overall thickness of thereflective polarizer layer, including the alternating microlayers, theprotective boundary layers and the skin layers, was approximately 40microns. The refractive indices, measured at 633 nm, for the alternating138 microlayers of 90/10 coPEN were n_(x1)=1.805, n_(y1)=1.620, andn_(z1)=1.515; and the indices for the 138 microlayers of FN007 weren_(x2)=n_(y2)=n_(z2)=1.506.

Optical diffuser 2850 was prepared using the method described inInternational Publication No. WO 2008/144656. The diffuser included aplurality of small particles dispersed in a binder. In particular, PMMAbeads (MBX-20, available from Sekisui) having an average diameter ofabout 18 micrometers were dispersed in a solution of Iragacure142437-73-01, IPA, and Cognis Photomer 6010 (available from Cognis NorthAmerica, Cincinnati, Ohio). The solution was coated on reflectivepolarizer layer 2860 and UV cured, resulting in a dried coatingthickness of approximately 40 microns. The dispersion of PMMA beadscreated a partial of hemispheric surface structure, randomly distributedspatially. The average radius of protrusion of the PMMA beads above themean surface was estimated to be approximately 60% of the average beadradius. The dried matrix was formulated to have approximately the samerefractive index as the PMMA beads, minimizing the bulk scatteringwithin the coating.

Optical performance of light source 2800 was measured using an AutronicConoscope Conostage 3 (available from Autronic-Melchers GmbH, Karlsruhe,Germany). LEDs 2830 were driven at 50 mA during the measurements. Theaxial luminance, maximum luminance, angles of maximum luminance (indegrees) along the x-axis (the down-lightguide direction) and the z-axis(the cross-lightguide direction) relative to the y-direction, andintegrated intensity were measured and summarized in Table I.

TABLE I Measured optical properties for Examples 1 and 2 Angle of Angleof maximum maximum Axial Maximum luminance luminance Integratedluminance luminance (z-axis) (x-axis) intensity Example (cd/m²) (cd/m²)(degrees) (degrees) (lm/m²) 1 1201.1 1214.8 63 270 2589.9 2 1274.41352.7 63 273 2890.8

Example 2

A light source similar to light source 2800 was made except that opticalstack 2890 was replaced with an optical stack 2900, a side-view of whichis shown schematically in FIG. 7.

Optical stack 2900 included 1.5 mm thick PC substrate 2880, OCA 8173optically clear adhesive layer 2870, reflective polarizer layer 2860having the same optical properties as the reflective polarizer layer inoptical stack 2890, an optical film 2940 coated on polarizer layer 2860,an optical diffuser 2910 similar to optical diffuser 2850 and coated ona substrate 2920, and an optical adhesive layer 2930 laminatingsubstrate 2920 to optical film 2940. Optical stack 2900 was placed atoutput port 2826 of optically reflective cavity 2810 in FIG. 6.

Optical film 2940 was made by coating solution A from Example A onreflective polarizer layer 2860 using the coating method described inExample B, except that the syringe pump rate was 6 cc/min and theUV-LEDs were ran at 13 Amps (resulting in a UVA dose of 0.135 joules persquare cm). The optical film had an index of refraction of about 1.22and a thickness of about 5 microns.

Optical diffuser 2910 was prepared using the method described inInternational Publication No. WO 2008/144656. The diffuser included aplurality of small particles dispersed in a binder. In particular, PMMAbeads (MB30X-8, available from Soken Chemical Company, Ltd, Tokyo Japan)having an average diameter of about 8 micrometers (22.5% by weight) weremixed with Cognis 6010 resin (15% by weight) (available as Photomer 6010from Cognis North America, Cincinnati Ohio), photoinitiator Esacure(0.1% by weight) (available from Lamberti S.p.A., Gallarate, Italy),radiation curing silicone additive Tego Rad 2250 (0.1% by weight)(available from Evonik Goldschmidt Corporation, Hopewell Va.), andsolvent Dowanol PM (61.9% by weight) (available from Dow ChemicalCompany, Midland Mich.). The components were mixed in a high shear mixerwith the beads added last. The solution was coated on a 0.051 mm thickPET substrate 2920, dried and UV cured, resulting in a dried coatingthickness of approximately 8 microns.

Optical performance of light source 2800 was measured using theprocedure described in Example 1. The results are summarized in Table I.

Example 3

A display system 800, a side-view of which is schematically shown inFIG. 8, was made. Display system 800 included a rectangular liquidcrystal panel 820 disposed on an extended light source 801. Liquidcrystal panel 820 had a length (x-direction) of about 895 mm and width(z-direction) of about 515 mm. The extended light source 801 had arectangular emissive or light emitting area that was 705 mm long(x-direction) and 400 mm wide (z-direction). Extended light source 801illuminated a similar size area of liquid crystal panel 820 that wassmaller than the size of the panel.

Light source 801 included an optical stack 890 that was disposed on andreceived light from an optically reflective hollow cavity 810. Theoptical cavity had a proximal side reflector 824, a distal sidereflector 820, a bottom reflector 822, and a lamp source assembly 870that included six light engines. Each light engine included a lamp 830that was housed within a parabolic reflector 840. Each lamp 830 included12 cool white LEDs (available under the name Luxeon Rebel from PhilipsLumiled Lighting Company, San Jose, Calif.) arranged in a linear arraywith a pitch of about 9.8 mm. The light engines were attached toaluminum heat sinks for thermal management. The optical cavity had thefollowing dimensions: l₁=1 mm, l₂=17 mm, l₃=400 mm, l₄=17 mm, and l₅=21mm. The interiors of the parabolic reflector, the side reflectors, andthe bottom reflector were lined with ESR mirror films (available as from3M Company, St. Paul Minn.) having a 99.5% reflectance in the visible.Optically reflective hollow cavity 810 had an open output port 826 fortransmitting light that was emitted by lamps 830.

Optical stack 890 included an optical diffuser 850 coated on areflective polarizer layer 860. The reflective polarizer was bonded toliquid crystal panel 820 via an optical adhesive layer 870. Reflectivepolarizer layer 860 was similar to reflective polarizer layer 2860 andwas made as described in Example 1.

Optical diffuser 850 was prepared as follows: 15 kg of Photomer 6010(available from Cognis USA, Cincinnati, Ohio) and 62.1 kg of1-methoxy-2-propanol were combined under rapidly stirring until thePhotomer 6010 was completely dissolved. Then, 0.1 Kg of Tego Rad 2250(available from Evonik Goldschmidt Corp. Hopewell, Va.), 0.53 kg ofEsacure ONE (available from Lamberti, Conshohocken, Pa.), and 22.5 kg ofMB30X-8 (available from Sekisui Plastics Co, Ltd. Tokyo, Japan) wereadded and rapidly stirred until a homogenous coating solution wasobtained. The resulting solution was then coated on reflective polarizerlayer 860 using a coating pump at a pump rate of about 800 g/min withthe reflective polarizer layer moving at about 30.5 m/min. Next, thecoating was dried by passing it through a first oven at 160° F. and asecond oven at 200° F. The dried coating was then UV cured using a LightHammer 6 UV light source that included H bulbs (available from Fusion UVSystems, INC. Gaithersburg, Md.) and operated at 100% UV under nitrogen.The resulting coated reflective polarizer layer was laminated to liquidcrystal panel 820 via optical adhesive layer 870 (adhesive OCA 8173available from 3M Company, St. Paul Minn.).

Optical performance of display system 800 was measured using EZ ContrastXL 88W Conoscope (Model XL88W-R-111124, available from Eldim-Optics,Hérouville Saint-Clair France). The display system had an axialluminance of about 93 nits, a maximum luminance of about 100 nits, acontrast ratio of about 146, a viewing angle of about 63 degrees alongthe x-axis and a viewing angle of about 30 degrees along the z-axis.FIG. 9 is a grayscale conoscopic image of the measured luminance ofdisplay system 800 as a function of viewing angle. The grid overlayingthe image is provided for reference purposes to show the azimuthal angleφ ranging from 0 to 360 degrees, and the polar angle θ ranging from 0 atthe center to about 88 degrees at the periphery, with concentric circlesprovided for each 20 degree increment of θ.

Example 4

A display system similar to display system 800 was made except thatoptical stack 890 was replaced with an optical stack 1090, a side-viewof which is shown schematically in FIG. 10. Optical stack 1090 includedoptical diffuser 850 coated on the bottom major surface of reflectivepolarizer layer 860, optical film 1010 coated on the top major surfaceof reflective polarizer layer 860, and optical adhesive layer 870.

Optical film 1010 was prepared and coated on reflective polarizer layer860 as follows. In a 2 liter three-neck flask, equipped with a condenserand a thermometer, 960 grams of IPA-ST-UP organosilica elongatedparticles (available from Nissan Chemical Inc., Houston, Tex.), 19.2grams of deionized water, and 350 grams of 1-methoxy-2-propanol weremixed under rapid stirring. The elongated particles had a diameter in arange from about 9 nm to about 15 nm and a length in a range from about40 nm to about 100 nm. The particles were dispersed in a 15.2% wt IPA.Next, 22.8 grams of Silquest A-174 silane (available from GE AdvancedMaterials, Wilton, Conn.) was added to the flask. The resulting mixturewas stirred for 30 minutes. The mixture was kept at 81° C. for 16 hours.Next, the solution was allowed to cool down to room temperature. Next,about 950 grams of the solvent in the solution were removed using arotary evaporator under a 40° C. water-bath, resulting in a 42.1% wtA-174-modified elongated silica clear dispersion in1-methoxy-2-propanol.

Next, 95 grams of this clear dispersion, 26.8 grams of SR 444 (availablefrom Sartomer Company, Exton, Pa.), 102 grams of isopropyl alcohol,0.972 grams of photoinitiator Irgacure 184 and 0.167 grams ofphotoinitiator Irgacure 819 (both available from Ciba SpecialtyChemicals Company, High Point N.C.) were mixed together and stirredresulting in a homogenous coating solution with 30.4% wt solids. Next,the coating solution was coated on the top major surface of reflectivepolarizer layer 860 using the coating method described below:

The coating solution was syringe-pumped at a rate of 2.5 cc/min into a20.3 cm wide slot-type coating die. The slot coating die uniformlydistributed a 20.3 cm wide coating onto a substrate moving at 152cm/min. Next, the coating was polymerized by passing the coatedreflective polarizer layer through a UV-LED cure chamber that included aquartz window to allow passage of UV radiation. The UV-LED bank includeda rectangular array of 352 UV-LEDs (available from Cree, Inc., Durham,N.C.), 16 down-web (coating direction) by 22 cross-web (approximatelycovering a 20.3 cm×20.3 cm area). The UV-LEDs were placed on twowater-cooled heat sinks. The UV-LEDs operated at a nominal wavelength of395 nm and were run at 45 Volts at 13 Amps, resulting in a UV-A dose ofabout 0.1352 joules per square cm. The UV-LED array was powered andfan-cooled by a Lambda GENH 60-12.5-U power supply (available fromTDK-Lambda, Neptune N.J.). The UV-LEDs were positioned above the curechamber quartz window at a distance of approximately 2.54 cm from thereflective polarizer layer. The UV-LED cure chamber was supplied with aflow of nitrogen at a flow rate of 46.7 liters/min resulting in anoxygen concentration of approximately 150 ppm in the cure chamber.

After being polymerized by the UV-LEDs, the solvent in the cured coatingwas removed by transporting the coating on a web to a drying ovenoperating at 150° F. for 2 minutes at a web speed of about 152 cm/mim.Next, the dried coating was post-cured using a Fusion System Model 1300Pconfigured with an H-bulb (available from Fusion UV Systems,Gaithersburg Md.). The UV Fusion chamber was supplied with a flow ofnitrogen that resulted in an oxygen concentration of approximately 50ppm in the chamber.

The resulting optical film 1010 had a thickness of about 5 microns andan effective refractive index of 1.16 measured using a Metricon Model2010 Prism Coupler (available from Metricon Corp., Pennington, N.J.). Asimilar optical film 1010 coated on a 0.051 mm thick PET substrate had atotal optical transmittance of about 94.9% and an optical haze of about1.1% as measured with a Haze-Gard Plus haze meter (available fromBYK-Gardiner, Silver Springs Md.).

The display system had an axial luminance of about 302 nits, a maximumluminance of about 312 nits, a contrast ratio of about 555, all of whichwere more than three times greater than the corresponding measurementsreported in Example 3. The display system had a viewing angle of about63 degrees along the x-axis and a viewing angle of about 35 degreesalong the z-axis. FIG. 11 is a grayscale conoscopic image of themeasured luminance of the display system as a function of viewing angle.

Item 1 is a light source comprising:

-   -   an optically reflective hollow cavity comprising:        -   one or more input ports for receiving light and an output            port for transmitting light; and        -   one or more lamps disposed at the one or more input ports;            and    -   an optical stack disposed at the output port and comprising:        -   a substantially forward scattering optical diffuser disposed            at the output port and having an optical haze that is not            less than about 20%;        -   an optical film disposed on the substantially forward            scattering optical diffuser for enhancing total internal            reflection at an interface between the optical film and the            substantially forward scattering optical diffuser, the            optical film having an index of refraction that is not            greater than about 1.3 and an optical haze that is not            greater than about 5%; and        -   a reflective polarizer layer disposed on the optical film,            wherein substantial portions of each two neighboring major            surfaces in the optical stack are in physical contact with            each other.

Item 2 is the light source of item 1, wherein a ratio of a maximumlateral dimension of the optically reflective hollow cavity to a maximumthickness of the optically reflective cavity is not less than about 20.

Item 3 is the light source of item 1, wherein a ratio of a maximumlateral dimension of the optically reflective hollow cavity to a maximumthickness of the optically reflective hollow cavity is not less thanabout 40.

Item 4 is the light source of item 1, wherein a ratio of a maximumlateral dimension of the optically reflective hollow cavity to a maximumthickness of the optically reflective hollow cavity is not less thanabout 60.

Item 5 is the light source of item 1, wherein the one or more lampscomprises one or more LEDs.

Item 6 is the light source of item 1, wherein the one or more inputports are located on opposite sides of the optically reflective hollowcavity and the output port is located on a top side of the opticallyreflective hollow cavity.

Item 7 is the light source of item 1, wherein the substantially forwardscattering optical diffuser has a transport ratio that is not less thanabout 0.2.

Item 8 is the light source of item 1, wherein the substantially forwardscattering optical diffuser has a transport ratio that is not less thanabout 0.3.

Item 9 is the light source of item 1, wherein the substantially forwardscattering optical diffuser has a transport ratio that is not less thanabout 0.4.

Item 10 is the light source of item 1, wherein the substantially forwardscattering optical diffuser has a transport ratio that is not less thanabout 0.5.

Item 11 is the light source of item 1, wherein the substantially forwardscattering optical diffuser comprises a semi-specular partial reflector.

Item 12 is the light source of item 1, wherein the optical haze of thesubstantially forward scattering optical diffuser is not less than about30%.

Item 13 is the light source of item 1, wherein the optical haze of thesubstantially forward scattering optical diffuser is not less than about40%.

Item 14 is the light source of item 1, wherein the substantially forwardscattering optical diffuser comprises a substantially forward scatteringsurface diffuser.

Item 15 is the light source of item 1, wherein the substantially forwardscattering optical diffuser comprises a substantially forward scatteringvolume diffuser.

Item 16 is the light source of item 1, wherein the substantially forwardscattering optical diffuser comprises a light scattering layer disposedon an optically transparent substrate.

Item 17 is the light source of item 1, wherein the effective index ofrefraction of the optical film is not greater than about 1.25.

Item 18 is the light source of item 1, wherein the effective index ofrefraction of the optical film is not greater than about 1.2.

Item 19 is the light source of item 1, wherein the effective index ofrefraction of the optical film is not greater than about 1.15.

Item 20 is the light source of item 1, wherein the effective index ofrefraction of the optical film is not greater than about 1.1.

Item 21 is the light source of item 1, wherein the optical haze of theoptical film is not greater than about 4%.

Item 22 is the light source of item 1, wherein the optical haze of theoptical film is not greater than about 3%.

Item 23 is the light source of item 1, wherein the optical haze of theoptical film is not greater than about 2%.

Item 24 is the light source of item 1, wherein the optical filmcomprises a plurality of interconnected voids.

Item 25 is the light source of item 1, wherein the optical filmcomprises a binder and a plurality of interconnected voids.

Item 26 is the light source of item 1, wherein the optical filmcomprises a binder, a plurality of interconnected voids, and a pluralityof particles.

Item 27 is the light source of item 1, wherein the optical film islaminated to the substantially forward scattering optical diffuser viaan optical adhesive layer.

Item 28 is the light source of item 1, wherein the optical film iscoated on the reflective polarizer layer.

Item 29 is the light source of item 1, wherein the optical stackcomprises an optically adhesive layer disposed on the reflectivepolarizer layer.

Item 30 is the light source of item 1, wherein the reflective polarizerlayer comprises a multilayer optical film comprising alternating layers,wherein at least one of the alternating layers comprises a birefringentmaterial.

Item 31 is the light source of item 1, wherein the reflective polarizerlayer comprises a wire grid reflective polarizer.

Item 32 is the light source of item 1, wherein the reflective polarizerlayer comprises a plurality of substantially parallel fibers, the fiberscomprising a birefringent material.

Item 33 is the light source of item 1, wherein the reflective polarizerlayer comprises a cholesteric reflective polarizer.

Item 34 is the light source of item 1, wherein the reflective polarizerlayer comprises a diffusely reflective polarizing film (DRPF).

Item 35 is the light source of item 1, wherein the optically reflectivehollow cavity comprises one or more specularly reflective sidereflectors, light that is emitted by the one or lamps being collimatedby the one or more specularly reflective side reflectors along a lateraldirection of the optically reflective hollow cavity.

Item 36 is the light source of item 1, wherein the optically reflectivehollow cavity comprises a specularly reflective reflector facing theoutput port.

Item 37 is the light source of item 36, wherein the output port issmaller than the specularly reflective reflector.

Item 38 is the light source of item 1, wherein the optical reflectivehollow cavity comprises one or more specular reflectors.

Item 39 is the light source of item 38, wherein the one or more specularreflectors include one or more enhanced specular reflectors (ESRs).

Item 40 is the light source of item 1, wherein at least 50% of each twoneighboring major surfaces in the optical stack are in physical contactwith each other.

Item 41 is the light source of item 1, wherein at least 70% of each twoneighboring major surfaces in the optical stack are in physical contactwith each other.

Item 42 is the light source of item 1, wherein at least 90% of each twoneighboring major surfaces in the optical stack are in physical contactwith each other.

Item 43 is the light source of item 1, wherein the optical film isdisposed between the reflective polarizer layer and the substantiallyforward scattering optical diffuser.

Item 44 is a backlight for providing illumination in a display system,the backlight comprising the light source of item 1.

Item 45 is a display system comprising the light source of item 1 and aliquid crystal panel disposed on the optical stack.

Item 46 is the display system of item 45, wherein the optical stack isbonded to the liquid crystal panel via a removable adhesive.

Item 47 is the display system of item 45, wherein the optical stack isbonded to the liquid crystal panel via a repositionable adhesive.

Item 48 is the light source of item 1, wherein the optically reflectivehollow cavity further comprises an optical element disposed near aninput port in the one or more input ports, the optical elementcomprising an optical filter, an asymmetric optical diffuser, awavelength converter, or a light collimator.

Item 49 is a tiled light source comprising a plurality of light sourcetiles, at least one of the plurality of light source tiles comprisingthe light source of item 1.

Item 50 is a display system comprising the tiled light source of item49.

Item 51 is the display system of item 50 comprising a monolithic imageforming panel.

Item 52 is the display system of item 50 comprising a tiled imageforming panel.

Item 53 is a light source comprising:

-   -   an optically reflective hollow cavity comprising:        -   one or more input ports for receiving light and an output            port for transmitting light; and        -   one or more lamps disposed at the one or more input ports;            and    -   an optical stack disposed at the output port and comprising:        -   a substantially forward scattering optical film disposed at            the output port and having an optical haze that is not less            than about 30%; and        -   a reflective polarizer layer disposed on the optical film,            wherein substantial portions of each two neighboring major            surfaces in the optical stack are in physical contact with            each other.

Item 54 is the light source of item 53, wherein a ratio of a maximumlateral dimension of the optically reflective hollow cavity to a maximumthickness of the optically reflective hollow cavity is not less thanabout 20.

Item 55 is the light source of item 53, wherein a ratio of a maximumlateral dimension of the optically reflective hollow cavity to a maximumthickness of the optically reflective hollow cavity is not less thanabout 40.

Item 56 is the light source of item 53, wherein a ratio of a maximumlateral dimension of the optically reflective hollow cavity to a maximumthickness of the optically reflective hollow cavity is not less thanabout 60.

Item 57 is the light source of item 53, wherein the one or lampscomprise one or more LEDs.

Item 58 is the light source of item 53, wherein the one or more inputports are located on opposite sides of the optically reflective hollowcavity and the output port is located on a top side of the opticallyreflective hollow cavity.

Item 59 is the light source of item 53, wherein the substantiallyforward scattering optical film has a transport ratio that is not lessthan about 0.2.

Item 60 is the light source of item 53, wherein the substantiallyforward scattering optical film has a transport ratio that is not lessthan about 0.3.

Item 61 is the light source of item 53, wherein the substantiallyforward scattering optical film has a transport ratio that is not lessthan about 0.4.

Item 62 is the light source of item 53, wherein the substantiallyforward scattering optical film has a transport ratio that is not lessthan about 0.5.

Item 63 is the light source of item 53, wherein the optical haze of thesubstantially forward scattering optical film is not less than about40%.

Item 64 is the light source of item 53, wherein the optical haze of thesubstantially forward scattering optical film is not less than about50%.

Item 65 is the light source of item 53, wherein the substantiallyforward scattering optical film comprises a plurality of interconnectedvoids.

Item 66 is the light source of item 53, wherein the substantiallyforward scattering optical film comprises a binder and a plurality ofinterconnected voids.

Item 67 is the light source of item 53, wherein the substantiallyforward scattering optical film comprises a binder, a plurality ofinterconnected voids, and a plurality of particles.

Item 68 is the light source of item 53, wherein the substantiallyforward scattering optical film is laminated to the reflective polarizerlayer via an optical adhesive layer.

Item 69 is the light source of item 53, wherein the substantiallyforward scattering optical film is coated on the reflective polarizerlayer.

Item 70 is the light source of item 53, wherein the optical stackcomprises an optically adhesive layer disposed on the reflectivepolarizer layer.

Item 71 is the light source of item 53, wherein the reflective polarizerlayer comprises a multilayer optical film comprising alternating layers,wherein at least one of the alternating layers comprises a birefringentmaterial.

Item 72 is the light source of item 53, wherein the reflective polarizerlayer comprises a wire grid reflective polarizer.

Item 73 is the light source of item 53, wherein the reflective polarizerlayer comprises a plurality of substantially parallel fibers, the fiberscomprising a birefringent material.

Item 74 is the light source of item 53, wherein the reflective polarizerlayer comprises a cholesteric reflective polarizer.

Item 75 is the light source of item 53, wherein the reflective polarizerlayer comprises a diffusely reflective polarizing film (DRPF).

Item 76 is the light source of item 53, wherein the optically reflectivehollow cavity comprises one or more specularly reflective sidereflectors, light that is emitted by the one or lamps being collimatedby the one or more specularly reflective side reflectors along a lateraldirection of the optically reflective hollow cavity.

Item 77 is the light source of item 53, wherein the optically reflectivehollow cavity comprises a specularly reflective reflector facing theoutput port.

Item 78 is the light source of item 53, wherein the optical reflectivehollow cavity comprises one or more specular reflectors.

Item 79 is the light source of item 53, wherein the one or more specularreflectors include one or more enhanced specular reflectors (ESRs).

Item 80 is the light source of item 53, wherein at least 50% of each twoneighboring major surfaces in the optical stack are in physical contactwith each other.

Item 81 is the light source of item 53, wherein at least 70% of each twoneighboring major surfaces in the optical stack are in physical contactwith each other.

Item 82 is the light source of item 53, wherein at least 90% of each twoneighboring major surfaces in the optical stack are in physical contactwith each other.

Item 83 is a backlight for providing illumination in a display system,the backlight comprising the light source of item 53.

Item 84 is a display system comprising the light source of item 53 and aliquid crystal panel disposed on the optical stack.

Item 85 is a light source comprising:

-   -   an optically reflective hollow cavity comprising:        -   one or more input ports for receiving light and an output            port for transmitting light; and        -   one or more lamps disposed at the one or more input ports;            and    -   an optical stack disposed at the output port and comprising:        -   a substantially forward scattering optical diffuser disposed            at the output port and having an optical haze that is not            less than about 20%;        -   an optical film disposed on the substantially forward            scattering optical diffuser for enhancing total internal            reflection at an interface between the optical film and the            substantially forward scattering optical diffuser, the            optical film having an index of refraction that is not            greater than about 1.3 and an optical haze that is not            greater than about 5%; and    -   a partially reflective partially transmissive layer disposed on        the optical film, wherein substantial portions of each two        neighboring major surfaces in the optical stack are in physical        contact with each other.

Item 86 is a light source comprising:

-   -   an optically reflective hollow cavity comprising:        -   one or more input ports for receiving light;        -   first and second output ports for transmitting light; and        -   one or more lamps disposed at the one or more input ports;            and    -   first and second optical stacks disposed at respective first and        second output ports, each optical stack comprising:        -   an optical film having an optical haze that is not less than            about 30%; and        -   a reflective polarizer layer disposed on the optical film,            substantial portions of each two neighboring major surfaces            in the optical stack being in physical contact with each            other.

Item 87 is a display system comprising:

-   -   a first liquid crystal panel disposed on the first optical stack        of the light source of item 86; and a second liquid crystal        panel disposed on the second optical stack of the light source        of item 86.

As used herein, terms such as “vertical”, “horizontal”, “above”,“below”, “left”, “right”, “upper” and “lower”, “top” and “bottom”,“clockwise” and “counter clockwise” and other similar terms, refer torelative positions as shown in the figures. In general, a physicalembodiment can have a different orientation, and in that case, the termsare intended to refer to relative positions modified to the actualorientation of the device. For example, even if display system 1200 inFIG. 1 is flipped as compared to the orientation in the figure,reflector 1212 is still considered to be a “bottom” reflector.

All patents, patent applications, and other publications cited above areincorporated by reference into this document as if reproduced in full.While specific examples of the invention are described in detail aboveto facilitate explanation of various aspects of the invention, it shouldbe understood that the intention is not to limit the invention to thespecifics of the examples. Rather, the intention is to cover allmodifications, embodiments, and alternatives falling within the spiritand scope of the invention as defined by the appended claims.

What is claimed is:
 1. A light source comprising: an opticallyreflective hollow cavity comprising: one or more input ports forreceiving light and an output port for transmitting light; and one ormore lamps disposed at the one or more input ports; and an optical stackdisposed at the output port and comprising: a substantially forwardscattering optical diffuser disposed at the output port and having anoptical haze that is not less than about 20% and a transport ratio thatis not less than about 0.2; an optical film disposed on thesubstantially forward scattering optical diffuser for enhancing totalinternal reflection at an interface between the optical film and thesubstantially forward scattering optical diffuser, the optical filmhaving an index of refraction that is not greater than about 1.3 and anoptical haze that is not greater than about 5%; and a reflectivepolarizer layer disposed on the optical film, wherein substantialportions of each two neighboring major surfaces in the optical stack arein physical contact with each other; wherein the optical film includes aplurality of voids.
 2. The light source of claim 1, wherein plurality ofvoids of the optical film include a plurality of interconnected voids.3. The light source of claim 1, further comprising a wavelengthconverter disposed within the optically reflective hollow cavity.
 4. Thelight source of claim 1, wherein the optically reflective hollow cavityfurther comprises a back reflector facing the output port.
 5. The lightsource of claim 4, wherein the reflective polarizer layer isnon-parallel to the back reflector.
 6. The light source of claim 4,wherein one or both of the reflective polarizer layer and the backreflector is non-planar.
 7. The light source of claim 4, wherein theoutput port is smaller than the back reflector.
 8. The light source ofclaim 1, wherein the optical stack further comprises a light redirectingfilm disposed on the reflective polarizer layer.
 9. A tiled light sourcecomprising a plurality of light source tiles that can be individuallycontrolled, at least one of the plurality of light source tilescomprising the light source of claim
 1. 10. The light source of claim 1,wherein the light source is configured to provide illumination forgeneral lighting applications.